Add initial skeleton for Dashboard application

Summary:
Ref T3583. General idea here is:

• Users will be able to create DashboardPanels, which are things like the jump nav, or a minifeed, or recent assigned tasks, or recent tokens given, or whatever else.
• The DashboardPanels can be combined into Dashboards, which select specific panels and arrange them in some layout (and maybe have a few other options eventually).
• Then, you'll be able to set a specific Dashboard for your home page, and maybe for project home pages. But you can also use Dashboards on their own if you just like dashboards.

My plan is pretty much:

• Put in basic infrastructure for dashboards (this diff).
• Add basic create/edit (next few diffs).
• Once dashboards sort of work, do the homepage integration.

This diff does very little: you can't create dashboards or panels yet, and thus there are no dashboards to look at. This is all skeleton code, pretty much.

Test Plan:
omg si purrfect

{F106367}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D8109