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: {F6369164}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20398