R1:067d12d7168a

Converted the pinned applications selector to a typeahead.

Summary: Fixes T11513. Previously the selector was just a giant dropdown which was just... just too much. Now there's a handy typeahead.

Test Plan:
Happy Path:
Go to `Settings -> Home Page -> Pin Application`, start typing in the form then select one of the options. Click on "Pin Application". The application should now be in the list.

Other paths:
- Type nothing into the box and submit, nothing should happen.
- Choose an application that is…