Allow “Mark as Solution” in group messages

[mcwumbly (Dave McClure)]

@mcwumbly

I’ll do some topic merging in a bit,

@Moin

The second one is about adding the solved/unsolved filter, which of course requires the solved plugin in group inboxes. But it’s a different request.

I decided to keep those two topics open and retitle them as they are both more specific requests, although they both depend on this one.