Flag rooms with unsent msgs

[ara4n]

We need a clear way on all 3 platforms to tell you which rooms have unsent msgs pending. (Or we should be aggressively retrying to send msgs once connectivity returns; in fact, this might be the better soln?)

[rubo77]

I prefer the second solution (retrying). Also an unsent message should be stored locally, even if you close the app and reopen, it should still be there.