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 already pinned. The list should stay the same.
• Type nonsense into the box and submit, nothing should happen.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: chad, Korvin, epriestley, yelirekim

Maniphest Tasks: T11513

Differential Revision: https://secure.phabricator.com/D16459