Summary:
See title. Adds PhabricatorDashboardInstall data object which scopes installs to objectPHID + applicationClass. This is because we already have a collision for user home pages and user profiles. Assume only one dashboard per objectPHID + applicationClass though at the database level.
Fixes T5076.
Test Plan: From dashboard view, installed a dashboard - success! Went back to dashboard view and uninstalled it!
Reviewers: chad, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T5076
Differential Revision: https://secure.phabricator.com/D9206
