Allow adding new website admins from chat

Context: https://civictechto.slack.com/archives/C4SHX39B2/p1540251642000100

This is how to create a user:
https://developer.wordpress.org/rest-api/reference/users/#create-a-user

This plugins allows us to use the API with tokens instead of browser cookies:
https://wordpress.org/plugins/api-bearer-auth/

This command should be restricted to a channel like `#organizing-priv`

The idea would be for any organizer to be able to do this:

```
toby website add-admin patcon patrick.c.connolly@gmail.com
```

The password would be auto-generated. Depending on flow, we might need to have the bot reply, saying that a password reset is required. (Seems this can't get sent programmaticaly via api)

Start by tracing the Hubot command handlers and review the WordPress REST API users endpoint together with the API Bearer Auth plugin. Confirm how channel restriction, generated credentials, and the password-reset limitation should work before implementation. Done means the specified chat command safely creates an administrator through the API and communicates the required next step.

Written by the indexing model from the issue text.