Pull some constants out of the Asana bridge

Summary: Ref T2852. Reduce the number of magical strings in use, and prepare the Asana bridge for eventual workspace/project support (a little bit).

Test Plan: Verified enriched links still work properly.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6301