Check only top-level groups on sign-up form username endpoint

What does this MR do and why?

Check only top-level groups on sign-up form endpoint

WHen a user signs up for GitLab, the sign-up form checks to see if the username they want is already taken. Currently it checks all group names to see if there is a conflict, however, there is only a conflict for top-level groups, not sub-groups.

This changes the endpoint to check only top-level groups, not subgroups. I have verified that new users can sign up with the username of an existing subgroup.

Database Review

This modifies a query to check for the existence of a namespace.

Query before this MR

SELECT "namespaces"."id",
       "namespaces"."name",
       "namespaces"."path",
       "namespaces"."owner_id",
       "namespaces"."created_at",
       "namespaces"."updated_at",
       "namespaces"."type",
       "namespaces"."description",
       "namespaces"."avatar",
       "namespaces"."membership_lock",
       "namespaces"."share_with_group_lock",
       "namespaces"."visibility_level",
       "namespaces"."request_access_enabled",
       "namespaces"."ldap_sync_status",
       "namespaces"."ldap_sync_error",
       "namespaces"."ldap_sync_last_update_at",
       "namespaces"."ldap_sync_last_successful_update_at",
       "namespaces"."ldap_sync_last_sync_at",
       "namespaces"."description_html",
       "namespaces"."lfs_enabled",
       "namespaces"."parent_id",
       "namespaces"."shared_runners_minutes_limit",
       "namespaces"."repository_size_limit",
       "namespaces"."require_two_factor_authentication",
       "namespaces"."two_factor_grace_period",
       "namespaces"."cached_markdown_version",
       "namespaces"."project_creation_level",
       "namespaces"."runners_token",
       "namespaces"."file_template_project_id",
       "namespaces"."saml_discovery_token",
       "namespaces"."runners_token_encrypted",
       "namespaces"."custom_project_templates_group_id",
       "namespaces"."auto_devops_enabled",
       "namespaces"."extra_shared_runners_minutes_limit",
       "namespaces"."last_ci_minutes_notification_at",
       "namespaces"."last_ci_minutes_usage_notification_level",
       "namespaces"."subgroup_creation_level",
       "namespaces"."max_pages_size",
       "namespaces"."max_artifacts_size",
       "namespaces"."mentions_disabled",
       "namespaces"."default_branch_protection",
       "namespaces"."max_personal_access_token_lifetime",
       "namespaces"."push_rule_id",
       "namespaces"."shared_runners_enabled",
       "namespaces"."allow_descendants_override_disabled_shared_runners",
       "namespaces"."traversal_ids",
       "namespaces"."organization_id"
FROM "namespaces"
WHERE "namespaces"."type" != 'Project'
  AND (lower(path) = 'atevans-group-test'
       OR lower(name) = 'atevans-group-test') LIMIT 1

Query after this change

SELECT "namespaces"."id",
       "namespaces"."name",
       "namespaces"."path",
       "namespaces"."owner_id",
       "namespaces"."created_at",
       "namespaces"."updated_at",
       "namespaces"."type",
       "namespaces"."description",
       "namespaces"."avatar",
       "namespaces"."membership_lock",
       "namespaces"."share_with_group_lock",
       "namespaces"."visibility_level",
       "namespaces"."request_access_enabled",
       "namespaces"."ldap_sync_status",
       "namespaces"."ldap_sync_error",
       "namespaces"."ldap_sync_last_update_at",
       "namespaces"."ldap_sync_last_successful_update_at",
       "namespaces"."ldap_sync_last_sync_at",
       "namespaces"."description_html",
       "namespaces"."lfs_enabled",
       "namespaces"."parent_id",
       "namespaces"."shared_runners_minutes_limit",
       "namespaces"."repository_size_limit",
       "namespaces"."require_two_factor_authentication",
       "namespaces"."two_factor_grace_period",
       "namespaces"."cached_markdown_version",
       "namespaces"."project_creation_level",
       "namespaces"."runners_token",
       "namespaces"."file_template_project_id",
       "namespaces"."saml_discovery_token",
       "namespaces"."runners_token_encrypted",
       "namespaces"."custom_project_templates_group_id",
       "namespaces"."auto_devops_enabled",
       "namespaces"."extra_shared_runners_minutes_limit",
       "namespaces"."last_ci_minutes_notification_at",
       "namespaces"."last_ci_minutes_usage_notification_level",
       "namespaces"."subgroup_creation_level",
       "namespaces"."max_pages_size",
       "namespaces"."max_artifacts_size",
       "namespaces"."mentions_disabled",
       "namespaces"."default_branch_protection",
       "namespaces"."max_personal_access_token_lifetime",
       "namespaces"."push_rule_id",
       "namespaces"."shared_runners_enabled",
       "namespaces"."allow_descendants_override_disabled_shared_runners",
       "namespaces"."traversal_ids",
       "namespaces"."organization_id"
FROM "namespaces"
WHERE "namespaces"."type" != 'Project'
  AND "namespaces"."parent_id" IS NULL
  AND (lower(path) = 'atevans-group-test'
       OR lower(name) = 'atevans-group-test') LIMIT 1

• Explanation of query before change on Postgres.ai
• Explanation of query after change on Postgres.ai

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Before	After

How to set up and validate locally

1. create a new group username-test-group
2. create a new subgroup username-test-subgroup
3. ensure signup is enabled in your gitlab.yml
4. sign out, or open an incognito tab
5. go to the sign up form: https://gdk.test:3443/users/sign_up
6. enter username-test-subgroup in the "Username" box
7. verify that the form shows "Username is available" as help text

Related to #438828 (closed)