introduce tests for the API

[TomasTomecek]

I did not have the heart to ask @IceWreck to write API tests for his PR #479

But we should start testing the API code as well. I'd start with the calls which are migrated to psql.

[lachmanfrantisek]

The Schemathesis can be helpful for some validation and for testing the deployment (see #368 ). I am not sure if we can use it for writting custom tests.

edit: some updates can be found in the issue

[TomasTomecek]

my expectation here is functional tests: store some objects in DB and make sure you're getting the expected data in the API responses