Draft: Resolve "Group-level protected branches - Backend create and show"

What does this MR do and why?

Closes #1188 (closed)

1. Update ProtectedBranchPolicy to handle both projects as well as groups.

2. Create Groups::ProtectedBranchesController similar to Projects::ProtectedBranchesController. This will be used by frontend to create new protected branches for a group.

3. Update Groups::Settings::RepositoryController and create an instance variable like @protected_branches = @group.protected_branches.order(:name).page(params[:page]). This will be used by the frontend to list the protected branches on the group settings page.

4. Update the show method of Projects::Settings::RepositoryController to return @protected_branches as well as @protected_group_branches where @protected_group_branches = @project.root_namespace.protected_branches.order(:name).page(params[:page]). Frontend can use the new variable @protected_group_branches to distinguish the protected branches defined at the group level and mark them as locked as per the UI. This should be behind a feature flag.

5. Add these helper methods to the projects model so that it can be used at other places.

   def group_protected_branches
     root_namespace.protected_branches
   end
   
   def all_protected_branches
     protected_branches + group_protected_branches
   end

6. Find and update the usage of project.protected_branches with project.all_branches at the required places in the codebase. This should be behind a feature flag.

7. FF group_protected_branches

8. GitLab Premium License