Add data auth restriction ability

[gracebrownecodes]

• [ ] Extend the user model to associate users with sites.
• [ ] Expose user-site association in the admin.
• [ ] Add a custom Avocado QueryProcessor that appends site filtering to queries.
• [ ] Modify the local Cilantro code to pass an additional GET parameter that will inform serrano of which QueryProcessor to use.
• [ ] Add a custom control to the local Cilantro code to toggle between aggregate and filtered counts, stats, graphs, etc.
• [ ] Upgrade to use a forthcoming Serrano version that facilitates the above two points.

Rough time-target is end of August.