Save invitation outcome to invitation message and make available to UI

The shareable of an invitation can only be opened when an invitation was accepted and we still subscribe to the shareable.

To be able to set canBeOpened to a proper value, the user's response needs to be recorded in the invitation's metadata.

For the group invitation protocol the response could be stored in InviteeProtocolEngine#onLocalAccept() at the same time as marking the invitation unavailable to answer, then retrieved in GroupManagerImpl#parseInvitationRequest().