Simplify channel selection using radio buttons

This takes a similar approach as in the previous commit. Since the
"training channel" doesn't really exist, we need some extra code
to pre-select it if a service is already in training mode. As in
the previous commit, I've removed a few non-critical test cases
where we really don't need to test exhaustively.

Note that we also need some specific code to avoid pre-selecting an
option for non-broadcast services, which only used to work by fluke:
we would try to populate the field with (False, None, 'all'), which
isn't a valid combination, so nothing was selected.

app/main/forms.py

@@ -2392,14 +2392,14 @@ class ServiceBroadcastAccountTypeField(GovukRadiosField):
 
 
 class ServiceBroadcastChannelForm(StripWhitespaceForm):
-    channel = ServiceBroadcastAccountTypeField(
+    channel = GovukRadiosField(
         'Emergency alerts settings',
         thing='mode or channel',
         choices=[
-            ("training-test-all", "Training mode"),
-            ("live-test-all", "Test channel"),
-            ("live-severe-all", "Live channel"),
-            ("live-government-all", "Government channel"),
+            ("training", "Training mode"),
+            ("test", "Test channel"),
+            ("severe", "Live channel"),
+            ("government", "Government channel"),
         ],
     )