fix: an open popup follows a network switch the background saves (closes #650)

When the background saves a network switch after the user approves a
site's request, or another popup page saves one, an open popup takes the
new network from the stored record inside a save, holding the lock, and
checks the record once at start-up for a switch saved before it
listened. The network it takes is not counted as the popup's own change,
even when that save fails to write. It then leaves the page as a switch
in Settings does. A switch in Settings is now made inside its own save
too, so one picked while a follow waits for the lock is made after it.
The popup never switches the network itself.

Model: opus-5-5

src/popup/viewRouter.js

@@ -39,8 +39,8 @@ function markViewRendered(view) {
 }
 
 // Begin a fresh page-load scope. The popup gets one by being loaded, and
-// asks for one when the user switches the network, because every screen
-// then on the page shows the previous network (views/settings.js); the
+// asks for one after a network switch, because every screen then on the page
+// shows the previous network (showNewNetwork() in views/settings.js); the
 // unit tests, which simulate several page loads against one module
 // instance, ask for one too.
 function resetRenderedViews() {