* Rename the 'master' API key to be called the 'system' API key
* Generate the key using the Python secrets module which is meant for this
* Remove some debugging helper code which will be obsoleted by the upcoming changes for session keys
thanks @downtownallday
* this invalidates all user_keys after TOTP status is changed for user
* after changing TOTP state, a login is required
* due to the forced login, we can't and don't need to store the code used for setup in `mru_code`
Intended to be the simplest auth possible: every time the service
starts, a random key is written to `/var/lib/mailinabox/api.key`. In
order to authenticate to the service, the client must pass the contents
of `api.key` in an HTTP basic auth header. In this way, users who do not
have read access to that file are not able to communicate with the
service.
