Add command to create users

[tdruez]

Following #359
We've implemented authentication but we have no simple way to create simple users with their API key.

The current workaround is to use createsuperuser and use drf_create_token. This is not documented.

[tdruez]

Documented at https://scancodeio.readthedocs.io/en/latest/command-line-interface.html#scanpipe-create-user-username