Overhaul app page

With VDG guidance and mockups, this commit aims to increase the
aesthetic qualities and usefulness of the Application page in the
following ways:

- Move all the metadata from their rather hidden location on the bottom
  of the page to much higher up on the page in a new layout
- Add a defined header area that contains entirely informational
  metadata and screenshots
- Make all the actionable metadata (e.g. "read documentation," "visit
  homepage," "report bug") into buttons
- Improve the license display by only showing the first two when there
  are a lot, with a "show more" button to see them all
- Make the main content area centered rather than left-aligned when the
  page is wide enough to have the content area's maximum width limited

BUG: 447403
BUG: 438920
FIXED-IN: 5.25