License header check

[Florian Hockmann]

We should automatically check that all source files have a license header.

This should ideally be done in the Cake build script so it can be done locally as part of the usual build and also on the build server.

It's probably enough to check that the license URL is contained in all source files (which we can identify by their file extension):
http://www.apache.org/licenses/LICENSE-2.0.

[Florian Hockmann]

This was already implemented by @farodin91 as a GitHub action in this commit.