Read-only database support (#1441)

Features.
- Adds a decorator, use_readonly_database, that does what you'd expect.  Uses a Django database router to get its job done.
- Configuration support: readonly_database_host and friends in the environments file, readonly stanza in database.py
- Backward compatible: if readonly db specified, still works with existing environment and database.py files
- Decorated reports, in-line reports, and videocheck

Tested on staging by running reports off of the utility machine, and in-line reports off of the appservers.  Ran netstat watching for the IP addresses of the readonly DB, like this:

![app1 stage](https://f.cloud.github.com/assets/621565/121607/56f18248-6dcd-11e2-842d-d008caf9e627.png)