Add JIRA doorkeeper and remarkup support

Summary:
Ref T3687. Adds a Doorkeeper bridge for JIRA issues, plus remarkup support. In particular:

• The Asana and JIRA remarkup rules shared most of their implementation, so I refactored what I could into a base class.
• Actual bridge implementation is straightforward and similar to Asana, although probably not similar enough to really justify refactoring.

Test Plan:

• When logged in as a JIRA-connected user, pasted a JIRA issue link and saw it enriched at rendering time.
• Logged in and out with JIRA.
• Tested an Asana link, too (seems I haven't broken anything).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6878