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 :)

This new version forbids re-linking an already linked file or message to
another issues.

FWIW the attached patch is the solution currently applied on the
bugs.python.org instance.

See also http://psf.upfronthosting.co.za/roundup/meta/issue267

Note that this patch changes the tracker and not core code.

Also I have a tracker where I can create a "meta" issue.
I can link other issues (group them) into the meta issue.
By updating the meta issue, the same message is sent to all
the grouped issues.

This is an alternate way of merging multiple issues and treating
them as a single issue.

I am not quite sure but this patch seems to break that use case.

I wonder if it would be better to add this as an example on the wiki
and link to this ticket.

I do like providing a ui for linking/unlinking messages
and providing an undo action for a spurious unlink.

Maybe that should go into the classic tracker.

One other thing to note:

http://psf.upfronthosting.co.za/roundup/meta/msg1458

reports that unlinking/linking changes the order of display of the 
messages. The message is displayed after linking as the newest
message in the issue. Even though the date stamp is the original
datestamp.

This makes it a bit confusing. Consider opening another ticket
for this problem if it persists after implementing these changes.