Prevent editing and deleting comments in locked conversations

Summary:
Ref T13289. This tightens up a couple of corner cases around locked threads.

Locking is primarily motivated by two use cases: stopping nonproductive conversations on open source installs (similar to GitHub's feature); and freezing object state for audit/record-keeping purposes.

Currently, you can edit or remove comments on a locked thread, but neither use case is well-served by allowing this. Require "CAN_INTERACT" to edit or remove a comment.

Administrators can still remove comments from a locked thread to serve "lock a flamewar, then clean it up", since "Remove Comment" on a comment you don't own is fairly unambiguously an administrative action.

Test Plan:

• On a locked task, tried to edit and remove my comments as a non-administrator. Saw appropriate disabled UI state and error dialogs (actions were disallowed).
• On a locked task, tried to remove another user's comments as an administrator. This works.
• On a normal task, edited comments normally.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13289

Differential Revision: https://secure.phabricator.com/D20551