Make initial setup easier and less error prone

[frankwiles]

The biggest mistakes new and old users make when setting up a new instance of the API are:

1. Not setting up a superuser account for themselves
2. Not setting up the Simpl User that their model service will use to connect to the API (or forgetting to make them is_staff)

TODO:

• [x] Create management command to set these up
• [x] Document them in the README