Add stdlib candidate handling

Integrate proper stdlib candidate handling API into #15.

## API methods
- [x] create a candidate
- [x] list candidates
- [x] accept or reject a candidate (for stdlib team members)
- [x] candidate approval by item uploader
- [x] describe a candidate

***
- [x] further filters for candidate list ([see here](https://github.com/maul-esel/ALD-API/pull/25#commitcomment-2728085))