Fix license header issues and improve tooling

[wborn]

There are some issues with the license headers in source code files that need to be fixed:

• There are many source code files from which the header is missing. It might also be an idea to add it to the TypeScript files.
• The reference to CONTRIBUTORS.txt in the header can be removed because this information is already part of the Git repository history.
• The Gradle build should check that the license headers are correct and not missing. This could for instance be done using the license-gradle-plugin.

This also applies to other repostories like console-android, console-ios.