Use real Dashboard Panels to render the default hard-coded homepage, not hacky fake panels

Summary:
Ref T13272. Currently, the hard-coded default homepage looks like a dashboard but is actually rendered completely manually.

This means that various panel rendering improvements we'd like to make (including better "Show More" behavior and better handling of overheated queries) won't work on the home page naturally: we'd have to make these changes twice, once for dashboards and once for the home page.

Instead, build the home page out of real panels. This turns out to be significantly simpler (I think the backend part of panels/dashboards is mostly on solid footing, the frontend just needs some work).

Test Plan:
Loaded the default home page, saw a thing which looked the same as the old thing. Changes I know about / expect:

• The headers for these panels are no longer linked, but they weren't colorized before so the links were hard to find. I plan to improve panel behavior for "find/more" in a followup.
• I've removed the "follow us on twitter" default NUX if feed is empty, since this seems like unnecessary incidental complexity.
• (Internal exception behavior should be better, now.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20333