Allow ApplicationTransaction comments to be edited and deleted

Summary:
Allows you to edit or delete comments in appplications which support ApplicationTransactions.

UI/UX stuff:

• The dialogs are rough but I want to do a dialog design pass more generally, @chad has some mocks.
• When you add new mentions via edit, they don't currently count as mentions. I'm not sure what I want to do about this.
• When you edit or delete a comment, we do not publish any notifications about it. I think this is reasonable.
• I didn't separate "delete" out versus "edit"; I assume it will be reasonably intuitive that deleting all the text deletes effectively deletes the comment. I also want to discourage deletion somewhat (we still show the transaction, just show that the comment has been deleted).

Test Plan:
Transaction view, note "Edit" and "Edited" links:

{F26914}

Edit view, has some design issues but I want to do a pass on dialogs in general:

{F26915}

History view:

{F26913}

Reviewers: vrana, btrahan, chad

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1082

Differential Revision: https://secure.phabricator.com/D4149