Dashboards - add ability to install dashboard as home

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