Fix weird subscribe+comment rendering

Summary:
Fixes T5146. When we're rendering a transaction group that includes a comment, we hide the "x added a comment" text, since it's implicit and obvious and cleans the UI up a little.

However, the way this works is really complicated and messy and created the T5146 issue after I made self-subscriptions have a lower priority than comments do.

Clean this code up so it makes a little more sense and gets this case right.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5146

Differential Revision: https://secure.phabricator.com/D9245