Make email server settings configurable in UI

Settings to configure email for Graylog need to be configured in `graylog.conf`. I would like to be able to configure and test this in the UI. That would improve the look and feel and would make it much easier to figure out how to configure. Now the configuration cycle is ab bit complicated (edit graylog.conf, restart server, try again to make an alert from scratch) and not state of the art.

Configuring in the UI would allow to validate the settings "on the fly", e.g. ensure all needed parameters are set and have valid stuff. A nice "send test mail" button would be a dream, as already is avaliable in the form for creating alerts.

see https://github.com/Graylog2/graylog-docker/issues/242

* Graylog Version: 5.0.2
* Elasticsearch Version: OpenSearch 2.4.8
* MongoDB Version: Mongo 5.0
* Operating System: dockerized
* Browser version: Vivaldi, newest