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.
Merge pull request #321 from 4teamwork/pg_assign_forwarding_fixes

Assign forwarding to dossier fixes