Relations API Endpoints

[JarJak]

Split from #163

Make API endpoints for relations introduced in #321.

TBD: I thought about something like /related/{from-id}/{to-contenttype}

[bobdenotter]

Bumping this to a later version. If there's a valid usecase, we can bump it higher on the priority list again.

[I-Valchev]

@bobdenotter Right, at some point this got created because it exists now. I'll go ahead and close this.