Implement POST /subject endpoint to create subjects

Note that this changes the existing endpoints "subjects/all" and "pools/all" to "subject/all", "pool/all" for consistency (new userstory requires "subject" endpoint and we used singular e.g. for questions as well). To be merged afer the PATCH /question mr since it builds on top of that. We also generally test non-ascii chars now. (can't get the ci/docker database to work with utf, works locally. Not that important i guess)