Verify that user can update branch on fork syncing

What does this MR do and why?

The MR consists of 3 smaller parts, but the parts are too small for separate MRs:

• Scope sync-fork feature flag to the fork source. It will allow us to enable the feature flag for all forks of a particular project (like gitlab-org/gitlab)
• Verify whether a branch is protected on fork syncing. It verifies that projectSyncFork mutation cannot be called if branch is protected.
• Display Update Fork button only if user can sync fork. Do not display the Update Fork button if branch is protected.

To test it locally

• Make sure that synchronize_fork feature flag is enabled
• Create a project and fork the project
• Then create a commit in the original project, so the fork is behind
• Visit the fork
• The Update Fork button must be displayed
• Protect the fork branch, so no-one can merge or push: https://docs.gitlab.com/ee/user/project/protected_branches.html#protected-branches
• The Update Fork button is not displayed