Add "JIRA Issues" field to Differential

Summary:
Ref T3687. This adds a field which allows you to link Differential Revisions to JIRA issues.

This is just about as basic as it can get, but gets the job done. The field enables itself if you have a JIRA auth provide. You enter JIRA issues in a comma-delimited format and it generates appropriate edges.

Nothing is pushed to the issues yet.

The only real rough part here is that if you commandeer a revision which is linked to issues you can't see, editing it is difficult via the CLI. This seems pretty much like a non-issue, but at some point we can let the field throw some kind of "RecoverableInvalidFieldException" which just warns the user. The "no reviewers, continue anyway?" prompt could then use that too.

Test Plan:

• Edited via web UI, tried valid/invalid edits, checked that edges showed up in the database, added/removed issues, clicked issue links.
• Edited via CLI, tried valid/invalid edits.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6879