apparently the nixpkgs mattermost module simply ignores its own config
if mutableConfig is false, and reads whatever config is currently in the
database (in our case, old stuff involving auth.infra4future.de).
This overwrites that.
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*
