Projects - fix missing subscriber handling code in the editor

Summary: Fixes T6719. At some point, we added automagical subscriptions via @mentions, and these were failing in project descriptions from a lack of an implementation in the editor. Said "implementation" is to do nothing, but it needs to be there nonetheless.

Test Plan: updated a project mentioning someone in the description and it worked. also saw a 'subscriber added' transaction

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6719

Differential Revision: https://secure.phabricator.com/D10951