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*
idea is to have a directory `websites/` which contains all our static
sites, with the name of each subdirectory also being their domain. Then
Nix can just read that directory during build-time and automatically
generate nginx virtualHosts for all of them (note that the
subdirectories have to contain a `default.nix` specifying how to build
the site for that to work).
Thus we could avoid the dependency on gitlab pages.
