Migrate from Maven to Gradle

[punycode]

Copied from #61:

Since CoffeeNet is currently a Spring Boot derived project, we try to track Spring Boot's upstream development. Recently Spring Boot switched (similar to Spring itself) to a Gradle based build and since CoffeeNet itself is modelled after the upstream build process, we might consider switching to a Gradle based build too.

This is may not be only for the sake of it, but could provide following advantages:

• Use the same proven build tooling as upstream Spring Boot
  • incremental builds
  • Spring (Boot) specific build enhancements
• Replace shell based custom build steps with Gradle ones, which can be tested more easily
• Simplify the Maven XML desert into something more accessible

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[punycode]

The automations for building and releasing with Gradle are in place. The only thing, we did not migrate, is the automatic update of external CoffeeNet projects to the current parent version. This was kind of broken anyway, so we will address this issue in a separate PR, since it doesn't negatively effect the building and release of CoffeeNet itself.