Allow users to customize applicaiton tile sizes

Summary:
See discussion in D4438. Allows users to customize application tiles, and implements generally reasonable defaults so they hopefully won't.

Sizes are "invisible" (internal only, used to hide admin apps from non-admins), "hidden" (hide by default, show after clicking "Show More Applications"), "show" (show a small square tile) and "full" (show a full-width tile with subtitle).

Test Plan:
Default view for a non-admin:

{F29375}

Adjusted settings, hidden:

{F29373}

Adjusted settings, shown:

{F29374}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4439