Add stdlib candidate handling

[maul-esel]

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

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• [x] further filters for candidate list (see here)

[maul-esel]

Still missing: /list filters.