Make dashboard application launchable

Summary:
Ref T4980. This isn't quite ready to unbeta yet, but it's good enough to be shown in the launch view.

Also, name it "Dashboards" in the UI.

Test Plan: Viewed launcher, saw Dashboards. Clicked it, got to dashboard main page.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4980

Differential Revision: https://secure.phabricator.com/D9650