Introduce "Curtain" views, panels, and extensions

Summary:
This opens up the new action column to have specialized rendering and behavior. Briefly:

• Converted applications (right now, only Paste) render a CurtainView to build the column content.
• This view uses new extensions to build panels (projects, subscribers, tokens).
• The panel extension code and rendering can be changed without breaking old stuff.

Minor changes:

• Token awards now load their tokens, for consistency/simplicity.
• Removed the rest of the "fork of" / "forked from" UI in Paste -- I essentially removed these features a while ago, and no one has complained.

Test Plan:
UI is a bit rough, but works, and it's going to get changed now anyway:

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15414