Inform the user when the connection is lost

[scastiel]

Already supposed to work that way, but it seems that in some situations we can make updates that disappear on reload. Need to investigate.

[simonwalsh]

I think we should prioritize this issue. At the moment, it's pretty easy to work on a disconnected board, which results on loss of work.

Ideally, the user shouldn't be able to CRUD anything when the connection is lost and she would be warned of that fact a bit more clearly.

Happy to help on this if you want!