Choose the DB (ie the platform dataset)

[Aleyasen]

Currently have a configuration file to set the choice of DB. This is too cumbersome. Add a feature that allows the user to choose between multiple dbs on their settings page. A link to the Settings page can be added at the top left of the screen, which is not very crowded.
Default for a new user should be the test database backend, so that a new user sees something interesting immediately. Default for a returning user should be the last DB they looked at.
Say somewhere at the top of the screen which db the user is looking at. (OK to use a nickname chosen by the user, as real names might be kind of awful.)