Add validation for provider SMS percentages

We could alternatively put the "add up to 100%" error on the page
using form-level errors [^1] and a custom flash message. Putting
the error on each field is slightly simpler and does make it clear
the issue is with all of the fields together.

[^1]: 22636b55ed

app/main/forms.py

@@ -1742,6 +1742,8 @@ class EstimateUsageForm(StripWhitespaceForm):
 
 class AdminProviderRatioForm(Form):
     def __init__(self, providers):
+        self._providers = providers
+
         # hack: https://github.com/wtforms/wtforms/issues/736
         self._unbound_fields = [
             (
@@ -1760,6 +1762,25 @@ class AdminProviderRatioForm(Form):
             for provider in providers
         })
 
+    def validate(self):
+        if not super().validate():
+            return False
+
+        total = sum(
+            getattr(self, provider['identifier']).data
+            for provider in self._providers
+        )
+
+        if total == 100:
+            return True
+
+        for provider in self._providers:
+            getattr(self, provider['identifier']).errors += [
+                'Must add up to 100%'
+            ]
+
+        return False
+
 
 class ServiceContactDetailsForm(StripWhitespaceForm):
     contact_details_type = RadioField(