parsemail: wrap parse_mail() in a single transaction

parsemail: wrap parse_mail() in a single transaction

Wrap the entire parse_mail() call in transaction.atomic() so that all
database writes from email parsing run inside a single transaction.
The existing transaction.atomic() blocks inside parse_mail() become
savepoints within this outer transaction. The series deduplication
retry logic continues to work since savepoint rollbacks are scoped
to their own savepoint.

This also ensures that any on_commit() callbacks registered by signal
handlers only fire after the full email has been parsed and all
patch/series associations are committed.

Signed-off-by: Robin Jarry <robin@jarry.cc>
Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Stephen Finucane <stephen@that.guru>
[stephenfin: Slight tweaks to the release notes]