also allow to react on group invitations, thereby fixes #116

otherwise the error message has no space to be shown

also rename forums to forum

only access MultiSets on UI thread, catch NoSuchElementException in case of inconsistent messagecounter state

In BriarUI we were injecting MessageCounterImpl instead of
MessageCounter, resulting in two different instances of
MessageCounterImpl in different places, effectively not having a
singleton any longer.

Adapt to changes in core from briar!1775