Add command to create fake worker run

Depends #1420 (closed)

The fake_worker_version django command must be completed by fake_worker_run with the following inputs:

• user email
• existing worker version ID

The User model must have a nullable foreign key towards Process, named private_process. It will allow creating worker runs owned by the user.

When a new fake worker run is requested, the private process of the user is either retrieved or created. Its corpus is the "My project" one from the user (or the first one where they are admin). Its mode is workers

Ideally, such processes should be excluded from ListProcess endpoint.

Of course, this is kind of a hack #todiscuss