Index and surface usage sites for Dashboards
Summary:
Depends on D20397. Ref T13272. Similar to the recent "where are Herald rules used" stuff, show which menus Dashboards are installed in.
This is mostly straightforward, except that I pulled some of the Herald logic into a parent class so it could be shared.
Test Plan:
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13272
Differential Revision: https://secure.phabricator.com/D20398