Add `with_relationships` query param

Partially fixes #838 (closed)

Works with pleroma!2456 (merged)

mentioned in issue #838 (closed)

mentioned in merge request pleroma!2456 (merged)

Actually, merging this into develop prior to pleroma!2456 (merged) would make PleromaFE develop fail with BE develop (thanks to wonderful restrictions in our implementation of OpenAPI support). So I guess I'll just bundle a build of this branch without having it merged to develop.

mentioned in commit pleroma@74912dd9

mentioned in merge request pleroma!2530 (merged)

changed the description

@shpuld is this okay to merge? because then i'd merge the three mrs

Yeah I made these changes, just wasn't sure if the BE is ready

BE is ready (with pleroma!2456 (merged) merged), as mentioned in the description. So, good to merge but only with pleroma!2456 (merged) and pleroma!2530 (merged) (which is the bundle of current MR).

mentioned in commit bc9ed364

merged