Adjust action strength of self subscribe/unsubscribe

Summary:
Ref T4968. If you add a comment to revision you aren't currently subscribed to, the email currently chooses "[Changed Subscribers]" as the action title. This is less interesting than "[Commented]", provided the affected subscriber is you (adding other people //is// usually interesting).

In this case, reduce the strength of this action below the strength of "comment".

Test Plan: Made several comments in conjunction with implicit and explicit subscriptions. Saw "[Commented]" for stuff affecting me, and "[Changed Subscribers]" for stuff affecting others.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4968

Differential Revision: https://secure.phabricator.com/D9168