CI/CD implementation via GH actions

[scrummer]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	yes
RFC?	yes/no

Since GH actions are released now (currently beta only afaik), this seems the best way to build the addons.
This has the advantage that we don't have a dependency to travis or sth and the integration seems quite good to me. This is why I created a test branch to play a little bit with the new feature scrummer-gh-actions