Add property to construct account type string

This allows us to start decoupling the form fields from the final,
hyphenated string, which we'll do in the next commits.

Note that I've also removed the conditional that changes the data
of the network field as part of validating it. We shouldn't change
data in validations, and having the new property directly above
makes it clear there's no need for this code.

app/main/forms.py

@@ -2430,11 +2430,18 @@ class ServiceBroadcastNetworkForm(StripWhitespaceForm):
         thing='a mobile network',
     )
 
+    @property
+    def account_type(self):
+        if self.network_variant.data == f'live-{self.broadcast_channel}-all':
+            provider = 'all'
+        else:
+            provider = self.network.provider_restriction
+
+        return f'live-{self.broadcast_channel}-{provider}'
+
     def validate_network(self, field):
         if not self.network_variant.data and not field.data:
             raise ValidationError('Select a mobile network')
-        if self.network_variant.data == 'all':
-            field.data = ''
 
 
 class ServiceBroadcastAccountTypeForm(StripWhitespaceForm):