Allow external diffs for merge requests to be configured

Omnibus part of gitlab-ce!24276

We're allowing merge request diffs to be stored on disk or in object storage, in the same way we do for artifacts or LFS objects. Unlike those, we're retaining the ability to store them in the database as well.

I initially had this in a fork, but it looks like the pipeline is broken for forks at present: !3014 (closed)