Move email configuration into cluster config

The server email configuration is currently done in the `graylog.conf` file and requires a server restart after changing it. We want to move this configuration into the cluster config so it can be changed from the UI and without restarting the server.

## Tasks

- [ ] Create cluster config object to store email configuration
- [ ] Write migration to store existing email configuration from `graylog.conf`/`server.conf` in the database
- [ ] Create UI component for the "System / Configurations" page to edit the email configuration
- [ ] Switch users of the config file settings over to the new cluster config based settings (e.g. alerting)