Cherry-pick any commit into any branch in the web interface

This would allow to cherry-pick any commit into an arbitrary branch:

• normal commit with git cherry-pick SHA
• merge commit with git cherry-pick -m 1 SHA

This, for instance, would ease the work and streamline the process described here: https://gitlab.com/gitlab-org/release-tools/blob/master/doc%2Fpatch.md#2-pick-specific-changes-into-the-stable-branches