Add remarkup support for Asana URIs

Summary:
Ref T2852. Primarily, this expands API access to Asana. As a user-visible effect, it links Asana tasks in Remarkup.

When a user enters an Asana URI, we register an onload behavior to make an Ajax call for the lookup. This respects privacy imposed by the API without creating a significant performance impact.

Test Plan: {F47183}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6274