Enable file locking for feature-branches

Description

File locking is an awesome EE-only feature, but it only locks files in the default branch.

Proposal

Make file locking available in feature-branches.

Links / references

We usually collaborate on code using branching strategies, so, being able to lock a file while you're working on it, preventing that other devs push changes at the same time as you're working on that file would prevent a lot of merge conflicts. This is the main purpose of file locking, right? But we barely push to the default branch directly.

Documentation blurb

We'd need to update https://docs.gitlab.com/ee/user/project/file_lock.html

@mydigitalself what do you think?