File lock should apply to all branches

Description

File locking (https://gitlab.com/gitlab-org/gitlab-ee/issues/497) currently only impacts the default branch, this means that anybody can edit the file on another branch, which seems counter-intuitive to the feature.

Proposal

File locking should apply to all branches, not just the default branch.

Links / references

https://gitlab.com/gitlab-org/gitlab-ee/issues/497

Documentation blurb

In GitLab 8.19, we released file locking, which is an incredibly useful way to prevent edits on a file. For reasons that seemed sensible at the time, a file could only be locked on the default branch.

In GitLab X.X the file lock now applies to all branches, which can prevent unexpected issues when merging and enforces the locked file on all other branches.