Prevent subquery on namespaces table

What does this MR do and why?

We already have the namespace object loaded so we can get the IDs via the traversal_ids column without an extra query. We can then pass these IDs directly to the where clause instead of building a namespace subquery.

This scope is used by Project#jira_subscription_exists?.

Query changes

Before:

SELECT 1 AS one
FROM "jira_connect_subscriptions"
WHERE "jira_connect_subscriptions"."namespace_id" IN (
  SELECT "namespaces"."id" FROM "namespaces" WHERE "namespaces"."type" = 'Group' AND "namespaces"."id" IN (107, 109)
)
LIMIT 1

After:

SELECT 1 AS one
FROM "jira_connect_subscriptions"
WHERE "jira_connect_subscriptions"."namespace_id" IN (109, 107)
LIMIT 1

MR acceptance checklist

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Related to #436078 (closed)