Allow users to add comments in Pholio

Summary: Basic support for adding comments. Missing a lot of frills. Uses new comment/transaction UI.

Test Plan:
Added some comments. Tried to add an empty comment.

Some comments:

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Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2097

Differential Revision: https://secure.phabricator.com/D3834