Fix: Revert masking the content field

Masking does not meaningfully improve privacy (the second half of the
content is stored in full), it does not save space (the asterisks are
stored as string in the database) and it breaks the expectation that the
auditlog can actually be used to audit changes.

This partly reverts #6388 (05b1ff9738201cafd7dea2cba4d9e5395b1e33da)

Resolves #6961

src/documents/models.py

@@ -929,7 +929,6 @@ if settings.AUDIT_LOG_ENABLED:
     auditlog.register(
         Document,
         m2m_fields={"tags"},
-        mask_fields=["content"],
         exclude_fields=["modified"],
     )
     auditlog.register(Correspondent)