Make default view of "Applications" app a full-page launcher

Summary:
This probably needs some tweaks, but the idea is to make it easier to browse and access applications without necessarily needing them to be on the homepage.

Open to feedback.

Test Plan:
(This screenshot merges "Organization", "Communication" and "Core" into a single "Core" group. We can't actually do this yet because it wrecks the homepage.)

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5176

Differential Revision: https://secure.phabricator.com/D9297