When an object is referenced by URI, treat it like a mention

Summary:
Ref T13291. Currently, T123 is a mention and adds an "alice mentioned this on Txxx." to T123, but https://install.com/T123 is not a mention.

Make the full URI a mention.

Test Plan: Commented a full URI, saw the target object get a mention story in its timeline.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13291

Differential Revision: https://secure.phabricator.com/D20527