Get rid of Option<Vec<_>> in api structs (fixes #2820)

[Nutomic]

Api tests are failing with "unknown" even after updating js client, not sure whats the problem.

[dessalines]

The error is most likely this, so an update, delete, or get_results returning zero results throwing an error: https://docs.diesel.rs/master/diesel/result/enum.Error.html#variant.NotFound

Also, I was only referring to removing optional from the responses. Forms that do create/update/delete operations should probably stay optional, otherwise the back-end might throw errors if they're missing. IE if you want to update a post, you shouldn't need to provide discussion languages if you aren't updating those.

[Nutomic]

Alright thats easier. There is also #[serde(default)] to make vec params optional but no need for that.