Merge branch 'async-new-branch-button' into 'master'

Load the "New Branch" button asynchronously

Here's how it looks like for logged in users:

![new_branch](/uploads/718e3d9016a50a0432b9541dde5ca74c/new_branch.gif)

And here it is for anonymous users (or when a new branch can't be created by a logged in user):

![new_branch_anonymous](/uploads/dce42ee6a7dd7708443cdfdb56389d46/new_branch_anonymous.gif)

See merge request !3855