Better documentation, require password, better ssh error handling

Utility for creating a new user account in postfixadmin, and setting account

values via Nextcloud.

## Running

```

  create-account:

    image: docker-create-account:0.1.0

    container_name: create-account

    environment:

      - NEXTCLOUD_ADMIN_USER=${NEXTCLOUD_ADMIN_USER}

      - NEXTCLOUD_ADMIN_PASSWORD=${NEXTCLOUD_ADMIN_PASSWORD}

      - POSTFIXADMIN_SSH_PASSWORD=${POSTFIXADMIN_SSH_PASSWORD}

      - DOMAIN=${DOMAIN}

      - CREATE_ACCOUNT_PASSWORD=${CREATE_ACCOUNT_PASSWORD}

    networks:

      - serverbase

    depends_on:

      - nextcloud

      - postfixadmin

```

## Usage

Send a `PUT` request to `example.com:9000/create-account`. Required parameters:

```

target_email

password

password_confirm

displayname

email_quota

fallback_email

nextcloud_quota

auth             # must be the same as the CREATE_ACCOUNT_PASSWORD env var (for client authorization)

target_email     # email to create (including domain)

password         # user password

password_confirm # user password confirmation

displayname      # user displayname

email_quota      # email storage quota in postfix

fallback_email   # email for password reset in Nextcloud

nextcloud_quota  # storage quota in Nextcloud

```

You can use the following command to make a request:

```

curl -X PUT https://example.com:9000/create-account \

    -d target_email=user@example.com -d password=pw -d password_confirm=pw \

    -d displayname=name -d email_quota=quota -d fallback_email=user@otherdomain.com \

    -d nextcloud_quota=-1

    -d auth=$CREATE_ACCOUNT_PASSWORD -d target_email=user@example.com -d \

    password=pw -d password_confirm=pw -d displayname=name -d email_quota=quota \

    -d fallback_email=user@otherdomain.com -d nextcloud_quota=-1

```

## Response values:

On failure, status code will be 4xx or 5xx and response body one of the following:

```

'{"success": false, "message": "username_forbidden"}'

'{"success": false, "message": "username_taken"}'

'{"success": false, "message": "internal_error"}'

{"success": false, "message": "username_forbidden"} # this username is not allowed

{"success": false, "message": "username_taken"} # this username is already taken

{"success": false, "message": "internal_error"} # failed to connect to postfixadmin or Nextcloud

{"success": false, "message": "no_auth"} # auth missing or wrong

```

class MyHandler(BaseHTTPRequestHandler):

    def do_PUT(self):

        if self.path == '/create-account' or self.path == '/create-account/':

            # TODO: require a password

            length = int(self.headers['Content-Length'])

            postvars = urllib.parse.parse_qs(self.rfile.read(length))

            postvars = {k.decode(): v[0].decode() for k, v in postvars.items()}

            print(f'Received request PUT /create-account, parameters {postvars}')

            if postvars['auth'] != os.environ['CREATE_ACCOUNT_PASSWORD']:

                self.respond(401, "no_auth")

                return

            self.create_account(postvars['target_email'], postvars['password'], postvars['password_confirm'],

                                postvars['displayname'], postvars['email_quota'], postvars['fallback_email'],

                return

        # create account via postfixadmin ssh

        with paramiko.SSHClient() as ssh:

            try:

                ssh.set_missing_host_key_policy(paramiko.MissingHostKeyPolicy)

                ssh.connect(hostname='postfixadmin', username='pfexec', password=os.environ['POSTFIXADMIN_SSH_PASSWORD'])

                    f'--name {shlex.quote(displayname)} --quota {shlex.quote(email_quota)} --active 1 --welcome-mail 0')

                print(stdout.read())

                print(stderr.read())

            except (ssh.SSHException, ssh.AuthenticationException, ssh.socket.error, ssh.BadHostKeyException) as e:

                print(e)

                data = json.dumps({'success': False, 'message': 'internal_error'})

                self.respond(500, data)

                return

        # Edit nextcloud account, set quota and email

        auth = HTTPBasicAuth(os.environ['NEXTCLOUD_ADMIN_USER'], os.environ['NEXTCLOUD_ADMIN_PASSWORD'])