Don't mark subscribes/mentions as "Lock Overridden"

Summary:
See PHI1209. When a task is in "Hard Lock" mode, it's still possible to apply some changes to it. Notably:

• You can subscribe/unsubscribe.
• You can mention it on another object.
• You can add a relationship from some other object to it (e.g., select it as a "Parent Task" for some other task).

Currently, these types of edits will show a "Lock Overridden" timeline emblem icon. However, they should not: you didn't override a lock to make these changes, they just bypass locks.

For now, special case these cases (self subscribe/unsubscribe + inverse edge edits) so they don't get the little icon, since I think this list is exhaustive today.

Some day we should modularize this, but we'd need code like this anyway (since TYPE_SUBSCRIBE is not modular yet), and this seems unlikely to cause problems even if it's a bit rough.

Test Plan:

• Hard-locked a task.
  • Subscribed/unsubscribed, mentioned, relationship'd it as a non-author. No timeline emblems.
• Soft-locked a task.
  • Subscribed/unsubscribed, mentioned, relationship'd it, no timeline emblems.
  • Clicked "Edit", answered "yes" to the override prompt, edited it. Got a timeline emblem.
• Added some comments and stuff to a normal non-locked task, no emblems.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20513