check for known bad files (zero bytes, etc) by hash

## extid / ExtidReviewBot

Deeper check of external identifiers.

Checks:
- duplication of entities: that no entities with the same identifiers already exist
- DOI numbers should be registered (check doi.org)

## style

Check release entity style.

- release_type, status, etc are set
- title doesn't end in a period
- at least one contributor

## URLs

Look at URLs and see if they look valid.

Checks:
- for "single page" springer domain (and others)
- no 'None' web.archive.org links (old bug)