show overall and/or per repo stats on repositories overview page

[DRiKE]

The lower part of the repo page should give a quick overview of things that (negatively) affect anything RPKI. Errors of certain severity regarding the objects in the repositories should be listed here, ideally with deeplinks to the actual node (CER/CRL/MFT/ROA) in the resource view.

The /repostats endpoint is a first effort in the backend/webservice to provide the error-to-node mappings, grouped by repository domain.

[DRiKE]

For the backend:

• put the remarks in a separate level under every repo, so we can add in other stats later as well
• the remark is incorrectly JSON-ified in the response