restore type message listener

[gkatrakazas]

I restored unregisterMessageListener type as it was causing problems with notifications. Additionally, it appeared that this type wasn't properly connected to the resolved error page.

[pstamatop]

I can't find the pattern of the behavior but with this change I got the unregisterMessageListener is not a function error.
However, on this branch the fcm token is saved on the database.

[emlun]

I think there are some issues in handleServerMessagesGuard - it currently opens a new socket on each render, for example. Maybe that's the cause of some of the issues.

I experimented a bit with restructuring handleServerMessagesGuard to make better use of the React state model, but that seemed to just surface other issues instead. I'll push the branch so you can take a look at it, though.