R1:73924dfa1812

Add initial skeleton for Dashboard application

Summary:
Ref T3583. General idea here is:

- Users will be able to create `DashboardPanel`s, which are things like the jump nav, or a minifeed, or recent assigned tasks, or recent tokens given, or whatever else.
- The `DashboardPanel`s can be combined into `Dashboard`s, 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…