Deploy to GitLab / course-pages automatically

[hroncok]

Currently a manual push to gitlab is needed. That is error prone.

Examine the following solutions:

1. make GitHub push to GitLab automatically (AFAIK not possible)
2. make GitLab fetch from GitHub automatically (last time I've checked it was not possible, cc @jirutka)
3. use a CI service to push to GitLab (tedious but indeed possible)

Most important: master to master.
Less important but nice to have: Pull Request to extra branch for preview.