API endpoint to create new users about pipalhub HOT 1 OPEN

pipalacademy/engage#62

I think we would need an endpoint that would allow us to create users for the above task in Engage. We could hackily add data to Engage and users.txt by hand but having an endpoint will let us automate the entire process.

Approach:

Authentication
Since this endpoint will create new users, this needs to have authentication. We don't have to implement our own, and we can use HubAuth to use the same authentication as the jupyterhub server. We can create a jupyterhub token as an admin user from the UI and set that as configuration in Engage. Once done, we only need to check whether the user making the request is an admin or not.

API
Create user

Successful request:

POST /services/dashboard/users
{
    "username": "alice",
    "password": "alice42"
}
---
201 CREATED
{
    "username": "alice"
}

Failed request:

POST /services/dashboard/users
{
    "username": "alice",
    "password": "hello:world"
}
---
400 BAD REQUEST
Content-Type: application/json
{
    "message": "Password is invalid"
}

Other error messages:

Username is invalid

Invalid request body

Implementation
Dashboard service should:

1. Validate the username/password strings - all characters should be in [ascii_letters + digits] only. (because punctuation/whitespace might not work well with the format of users.txt)
2. Write username:password\n to users.txt
3. Run add-users.py script (stored at /opt/jhub/add-users.py)
4. Return response

from pipalhub.