Let users select electoral wards of local authorities

If a library has groups, we should show a link instead of selecting the
group directly.

Then we can give the user the choice of selecting the whole of that
group, or specific areas within the group.

For now the only libraries we have with groups are local authorities,
which group electoral wards.

app/main/forms.py

@@ -2161,3 +2161,23 @@ class BroadcastAreaForm(StripWhitespaceForm):
         return cls(choices=[
             (area.id, area.name) for area in sorted(library)
         ])
+
+
+class BroadcastAreaFormWithSelectAll(BroadcastAreaForm):
+
+    select_all = govukCheckboxField('Select all')
+
+    @classmethod
+    def from_library(cls, library, select_all_choice):
+        instance = super().from_library(library)
+        (
+            instance.select_all.area_slug,
+            instance.select_all.label.text,
+        ) = select_all_choice
+        return instance
+
+    @property
+    def selected_areas(self):
+        if self.select_all.data:
+            return [self.select_all.area_slug]
+        return self.areas.data