Build and deploy website with GitLab CI

This merge request changes the way the DUNE website is built: Instead of notifying a special daemon on the web server about commits to master and then having that daemon fetch, build and deploy (which was the cause for the frequent problems with website updates), the website now gets built and deployed as part of pushing to master.

Moreover, the CI system recognizes special "triggered" builds and scheduled builds and does a full rebuild of the website in those cases, including all documentation. The triggered builds can be initiated by us here in Heidelberg, and there will be a nightly scheduled build.