Decuple versioning of distro_signatures.json from server version

[SchoolGuy]

As a developer
I want to be able to have less work with the json file maintenance
so that I can help my users to easier contribute to the project

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At the moment we maintain distro_signatures.json coupled to the server version of Cobbler. Thus we have a version of the file for every branch in Cobbler plus a file on cobbler.github.io. This is a lot of maintenance. After #39 is done, we could integrate a step in the CI that checks the format of the JSON.

This opens up the possibility for us to change to a format based versioning, instead of the current Cobbler-version based approach.

[SchoolGuy]

For this issue to be considered done we need to utilize the cobbler.github.io link for the signatures with the schema version inside the main repository.