Importer: Fix and accept posts with old UUID

Closes #3844
- If a post in an import JSON file has an old UUID format (eg
'52825818238f7'), we'll create a new UUID using uuid.v4() and the
creation timestamp as basis.
- Test not included. Reason: UUID validation is omnipresent,
creating/importing/editing a post to hold a wrong UUID therefore
impossible. In order to write a test, we’d have to overwrite APIs,
which seems excessive.