Calling CreateElement with any value on slim_output, including False, returns a slim output

The slim_output query parameter on CreateElement causes it to return just {"id": "..."} instead of a full element serializer for the newly created element, to get in theory a small performance boost as no more SQL queries or further work is required to return the response. We usually handle boolean parameters by treating case-insensitive false, 0 and the empty string, ignoring leading and trailing spaces, as false values, and everything else as true, which is very similar to how Python would parse a boolean from a string, but for slim_output, any value is True. This is causing trouble in base-worker as it sometimes sends slim_output=False (see Slack discussion).

Each time I did some benchmarking on this endpoint in previous issues, I always found that adding slim_output made no difference at all. We didn't try to add a slim_output on other endpoints either, and for performance, using bulk endpoints makes more sense anyway. We should consider dropping this parameter entirely.