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