Any user can unlink/link any files or messages from/to any issue.
Attached patch solves spurious unlinking and makes it possible to
restore unlinked files/msgs via the web UI.

However, linking a file or message to many issues is still possible. A
'linked' Boolean attribute could be used to block linking any
already-linked msg/file to another issue, while still allowing e.g. to
move a file from one issue to another.

Any feedback on the current patch or about how to fix the multiple
issues per file/msg problem is most welcome :)