this does a couple things:
- redo mattermost's secret config as an env file passed to systemd
- get rid of modules/mattermost.nix and use upstream module instead
- move some of the stuff in secret.json which don't need to be there
into nix (e.g. smtp port)
Also, I set the log level to ERROR in the env file. Mattermost doesn't
seem to respect it otherwise *shrug*