Get rid of the core repository and use media repositories as well.

[Fohlen]

As soon as possible I'd like to remove the media repository from core and pull it via the media repository feature introduced by @aschaeffer

[Croydon]

Do we not care about old Inexor versions not working anymore or do we somehow make sure to checkout some specific version of the media repositorys?

Note: This speeds up nightly packaging and uploading and saves space.

[Fohlen]

We could add a release tag for this as well. I wouldn't add "stability" as a priority for alpha releases though

[a-teammate]

already done in 4a09301bd2ab77b77cbf101927aa43752e77b2dd