Update addon backend to support sending Klaxon email

[eyeseast]

[mitchelljkotler]

API to send yourself an email:
https://github.com/MuckRock/documentcloud/blob/master/documentcloud/users/views.py#L190-L207

Getting the auth token for an add-on run:
https://github.com/MuckRock/documentcloud/blob/master/documentcloud/addons/models.py#L129-L141

Getting the token on the Squarelet side:
https://github.com/MuckRock/squarelet/blob/master/squarelet/users/viewsets.py#L102-L118

When making the API call, there is no way to tell what add-on it is from - you only have the user's token. That means when creating the token for Klaxon (we need a setting to specify the add-on ID for Klaxon), we need to add a permission to it (which we already support for the processing tokens). This permission can then be checked when sending the message (request.auth.get("permissions", [])).