[web] Update DisconnectedBar to display based on useNetworkConnected()

Summary:
Part of https://linear.app/comm/issue/ENG-6137/modify-disconnectedbar-so-it-pulls-from-connectivityinfo.

To begin, I'm going to update the web and native components to skip eg useShouldShowDisconnectedBar, etc. and just get the boolean they need (is the network connected or not) "directly" from useNetworkConnected on web and (state => state.connectivity.connected) on native.

I'll then go ahead and rename all of eg useDisconnectedBarVisibilityHandler to make it clear that they're no longer connected to the DisconnectedBar. I think we'll still want all the Redux actions and whatnot for the "Keyserver status"? page that @ginsu is working on, so I won't rip them out.

Test Plan:

1. Open local web app
2. Kill local keyserver
3. Observe that nothing happens:

4. Disconnect laptop from WiFi
5. Observe the DisconnectedBar:

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat, ginsu

Subscribers: inka, ginsu

Differential Revision: https://phab.comm.dev/D10469