R1:f4201593808d

Implement Asana and JIRA external links via HyperlinkEngineExtension, not separate Remarkup rules

Summary:
Depends on D20527. Ref T13291. Now that we have more flexible support for URI rewriting, use it for Doorkeeper URIs.

These are used when you set up Asana or JIRA and include the URI to an Asana task or a JIRA issue in a comment.

Test Plan:
- Linked up to Asana and JIRA.
- Put Asana and JIRA URIs in comments.
- Saw the UI update to pull task titles from Asana / JIRA using my OAuth credentials.