First version of "Quest Tracker" Home NUX

Summary:
Ref T5317. General idea here is that Dashboards are replacing the home page soon. We think they'll do a good job for normal users, but they aren't very good for new administrators who have just completed an install. In this case, any dashboard we put there by default will be empty and not very useful or helpful. It's also technically a bit messy to build objects by default.

Instead, give new administrators a "Quest Tracker" UI to help them get through things. When they're done with setup steps, they build a dashboard and install it to replace the home page. They can add install-specific welcome messages during this process, so hopefully this will also ease onboarding for non-administrator users.

For now, you have to go to Config > Welcome Screen explicitly to see this UI. We can tweak/test it for a bit before replacing the home page.

Test Plan:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5317

Differential Revision: https://secure.phabricator.com/D9660