Resolve "Group SAML - Check SSO status on Git activity"

What does this MR do?

Creates additional check that:

• verifies if user has active SSO session
• when group have enforced SSO enabled
• with additional git_activity_check setting
• while making git requests.

Screenshots (strongly suggested)

migration output:

up:

== 20210312174321 AddEnforcedGitCheckToSamlProvider: migrating ================
-- add_column(:saml_providers, :git_check_enforced, :boolean, {:default=>false, :null=>false})
   -> 0.0049s
== 20210312174321 AddEnforcedGitCheckToSamlProvider: migrated (0.0133s) =======

down:

== 20210312174321 AddEnforcedGitCheckToSamlProvider: reverting ================
-- remove_column(:saml_providers, :git_check_enforced)
   -> 0.0078s
== 20210312174321 AddEnforcedGitCheckToSamlProvider: reverted (0.0189s) =======

feature video: https://www.loom.com/share/b5f6cebf97ae4875892c8a579d0a4ad1

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #9152 (closed)