Add user-configurable city name

This adds a dialog to the GTFS upload page where the user may change the name for their city; if no name has been set yet, it defaults to the value from Django settings.
There's now an endpoint at `api/city-name` that returns the user's city, which is used by the indicator calculation page to set the city name for indicator job runs, and to fetch the latest indicator version for the user's city.

![image](https://cloud.githubusercontent.com/assets/960264/4773168/509f98e6-5b9f-11e4-812f-2f800ce1be1e.png)