Modernize pinned homepage applications settings

Summary:
Ref T4103. A few bits here:

• We have an ancient "tiles" preference which was just a fallback from 2-3 years ago. Throw that away.
• Modenize the other pinned stuff. We should likely revisit this after the next homepage update but I just left the actual defaults alone for now.
• Lightly prepare for global default editing.
• Add a "reset to defaults" option.

Test Plan:

• Pinned, unpinned, reordered and reset application homepage order.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16028