Resolve "Allow SSO enforcement in group settings for GitLab.com"

Adds enforced SSO feature flag and UI toggle (both independent)

Database checklist

• Conforms to the database guides

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body
• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)

When adding or modifying queries to improve performance:

• Included data that shows the performance improvement, preferably in the form of a benchmark
• Included the output of EXPLAIN (ANALYZE, BUFFERS) of the relevant queries

When adding foreign keys to existing tables:

• Included a migration to remove orphaned rows in the source table before adding the foreign key
• Removed any instances of dependent: ... that may no longer be necessary

When adding tables:

• Ordered columns based on the Ordering Table Columns guidelines
• Added foreign keys to any columns pointing to data in other tables
• Added indexes for fields that are used in statements such as WHERE, ORDER BY, GROUP BY, and JOINs

When removing columns, tables, indexes or other structures:

• Removed these in a post-deployment migration
• Made sure the application no longer uses (or ignores) these structures

General checklist

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides

/label database

Closes #5291 (closed)