Merge #8996: Network activity toggle

19f46f1 Qt: New network_disabled icon (Luke Dashjr)
54cf997 RPC/Net: Use boolean consistently for networkactive, and remove from getinfo (Luke Dashjr)
b2b33d9 Overhaul network activity toggle (Jonas Schnelli)
32efa79 Qt: Add GUI feedback and control of network activity state. (Jon Lund Steffensen)
e38993b RPC: Add "togglenetwork" method to toggle network activity temporarily (Jon Lund Steffensen)
7c9a98a Allow network activity to be temporarily suspended. (Jon Lund Steffensen)