nixos/mailman: refactor

- Add serve.enable option, which configures uwsgi and nginx to serve
  the mailman-web application;
- Configure services to log to the journal, where possible. Mailman
  Core does not provide any options for this, but will now log to
  /var/log/mailman;
- Use a unified python environment for all components, with an
  extraPackages option to allow use of postgres support and similar;
- Configure mailman's postfix module such that it can generate the
  domain and lmtp maps;
- Fix formatting for option examples;
- Provide a mailman-web user to run the uwsgi service by default
- Refactor Hyperkitty's periodic jobs to reduce repetition in the
  expressions;
- Remove service dependencies not related to functionality included in
  the module, such as httpd -- these should be configured in user config
  when used;
- Move static files root to /var/lib/mailman-web-static by default. This avoids
  permission issues when a static file web server attempts to access
  /var/lib/mailman which is private to mailman. The location can still
  be changed by setting services.mailman.webSettings.STATIC_ROOT;
- Remove the webRoot option, which seems to have been included by
  accident, being an unsuitable directory for serving via HTTP.
- Rename mailman-web.service to mailman-web-setup.service, since it
  doesn't actually serve mailman-web. There is now a
  mailman-uwsgi.service if serve.enable is set to true.