As an API user I want to be able to edit statements on Forms through wbeditentity in order to quickly add/modify/remove data.
Currently editing of statements on Forms is not supported via `wbeditentity`.
GIVEN I make a `wbeditentity` API request with the `"claims"` key set within a form's JSON
WHEN I submit the request
THEN the form's statements are modified accordingly
* statements are saved persistently
* edit summary shows up in recent changes/Lexeme history/watchlist
~~* What parts of it are already working?~~ `wbsetclaim` works, `wbeditentity` doesn't work at all.
In theory, once T201389 is solved, the only thing left to make `wbeditentity` work, is to add the existing `ChangeOpDeserializer` for Forms.