make applicationTransactionsCommentView have a "Login to comment." button if user is not logged in
Summary: okay title. other apps can get this by implementing shouldAllowPublic and set(ting)RequestURI on TransactionsCommentView. note i put some css inline -- let me know if that belongs someplace else or needs better design.
Test Plan: viewed a mock logged out and saw new button. used new button and ended up on the mock logged in with a clean URI.
Reviewers: epriestley, chad
Reviewed By: chad
CC: aran, Korvin
Maniphest Tasks: T2653
Differential Revision: https://secure.phabricator.com/D5266