Assign forwarding to dossier fixes

**Assign forwarding to dossier: reset issuer to current inbox.**
When assigning a forwarding to a dossier, which concerning multiple org_units but only one adminunit,
the issuer has to be reset to current inbox in the successor task.

**Accept task: Set default for newdossier`s title only for tasks.**
For forwardings the default is only `Inbox`, which does not make sense at all.

@deiferni please have a look.