WIP: Add titles to Issues, MRs, and Commits when they're referenced by a mentionable.

Now references to Merge Requests and Issues from Commits, MRs, and Issues all provide their title inline.

Where it used to say mentioned in merge request !1 it now says mentioned in merge request !1 (We’re not the most popular operating system on the planet). And similar for commits and issues.

And yes, in case you were wondering. This does break all the tests.

Before:

After:

Fixes #1784 (moved).

cc: @rspeicher @jschatz1