R1:33688c8a41eb

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