Make the "Install Dashboard" flow smoother

Summary:
Depends on D20362. Ref T13272. Currently, Dashboards have an "Install Dashboard" flow which is pretty janky and only allows you to install things to the home page.

Instead, allow users to install things to any valid target (home, favorites, portals, projects). This also provides URIs like dashboard/install/1/home/personal/ which allow you to link users to an "install a dashboard" page; this may or may not get used.

Test Plan: Installed dashboards on home, favorites, projects, and portals.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20364