Set up CI/hooks to render preview pages for pull requests

[sudo42]

The current configuration would allow us to easily deploy the current state for each pending merge request on a sub-path or subdomain so one can easily click a link and does not have to pull and build the site locally.

These deployments could get removed automagically (after 48 hours or when the request is closed) (whatever happens first).

We should require at least a button-press in some way of a "trusted person" (repository/group member) before deploying for external pull requests.