Disable upload to docker v2 repos via web UI and hammer
The "docker save" command currently creates a docker image in the V1 registry format. This is incompatible with docker registry v2.
Until "docker save" creates v2 data, we need to disable upload of docker images via the web UI and hammer. Additionally, we need to document this in the 2.5 release docs, and mention that users can spin up a private registry, push to that, and sync down in order to get custom docker content into Katello.
Merge pull request #5740 from komidore64/remove-docker-upload-content
refs #13453 - removing ability for docker content upload