Restrict pushes / merges to a protected branch to specific groups

• Closes #675 (closed) and https://gitlab.com/gitlab-org/gitlab-ce/issues/21153
• Related to #179 (closed)

Default State

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Tasks

• ee#675 !645 (merged) Restrict pushes/merges to specific groups
  • Implementation
    • Basic model-level implementation
    • Test / refactor
    • Rudimentary frontend (controller actions + autocomplete for the dropdown)
      • Groups that a project hasn't been shared with can't be selected
    • Look for edge cases
  • Questions
    • What are LDAP group links?
      • A GitLab group can be synced with one or more LDAP groups
      • The sync happens in an async task, an the LDAP group users are imported into the GitLab group
      • group.users on a GitLab group returns the LDAP group users as well (after the import task has run)
      • We check group.users for this feature, so we shouldn't need any additional work to support LDAP groups
  • CHANGELOG
  • Removing a group should remove the access
  • autocomplete/groups issue
  • Fix uniqueness validation
  • Assign to @alfredo1 for UI work
  • Wait for !581 (merged) to be merged
  • Rebase against EE master instead of !581 (merged)
  • Add feature specs
  • Implement frontend
  • Assign to endboss
  • Get backend + frontend reviewed
  • Wait for merge