Add more user management

[nginyc]

Fixes #111

Before testing, since Rafiki's v0.1.0 Docker images have not been published to Docker Hub, will need to run source .env.sh, then bash scripts/build_images.sh for every participating machine.

• Admins can ban users
• Admins can list all users
• Admins cannot ban other admins (only superadmins can do this)
• Admins cannot ban themselves
• Admins cannot create a user with an invalid type
• Add documentation on new user APIs

[nudles]

It is not common to create users using a csv file. Typically, we add users one by one. But we need to get all users for the front-end.
Have you compared users vs user for the operations against a single user? Both are fine, but we need to be consistent with other APIs. E.g., model vs models, trial vs trials.

[nginyc]

Noted. I will be removing this API

[nginyc]

Changed from deleting user to banning user to avoid complication of having to consider cascade-deleting all associated train jobs / trials / inference jobs / models under that user.

[wild-flame]

Can add to documentation:

for sudo users need to run

sudo -E bash scripts/build_images.sh after run source env.sh

according to https://stackoverflow.com/questions/8633461/how-to-keep-environment-variables-when-using-sudo to preserve the environment variable

[nginyc]

Thanks @wild-flame for the feedback. I've added that to the documentation.