Rewrite SMS ratio form to cope with 3 providers

This replaces the slider with an integer input for each provider.
Unfortunately showing a variable number of inputs isn't easy to
achieve in WTForms [^1], but we think this is the least worst way
to do it vs e.g. not using WTForms at all.

[^1]: https://github.com/wtforms/wtforms/issues/736

app/main/forms.py

@@ -1740,29 +1740,26 @@ class EstimateUsageForm(StripWhitespaceForm):
         return super().validate(*args, **kwargs)
 
 
-class AdminProviderRatioForm(StripWhitespaceForm):
+class AdminProviderRatioForm(Form):
+    def __init__(self, providers):
+        # hack: https://github.com/wtforms/wtforms/issues/736
+        self._unbound_fields = [
+            (
+                provider['identifier'],
+                GovukIntegerField(
+                    provider['display_name'],
+                    validators=[validators.NumberRange(
+                        min=0, max=100, message="Must be between 0 and 100"
+                    )]
+                )
+            ) for provider in providers
+        ]
 
-    ratio = GovukRadiosField(choices=[
-            (str(value), '{}% / {}%'.format(value, 100 - value))
-            for value in range(100, -10, -10)
-        ],
-        param_extensions={
-            "classes": "govuk-radios--inline",
-            "fieldset": {
-                "legend": {
-                    "classes": "govuk-visually-hidden"
-                }
-            }
+        super().__init__(data={
+            provider['identifier']: provider['priority']
+            for provider in providers
         })
 
-    @property
-    def percentage_left(self):
-        return int(self.ratio.data)
-
-    @property
-    def percentage_right(self):
-        return 100 - self.percentage_left
-
 
 class ServiceContactDetailsForm(StripWhitespaceForm):
     contact_details_type = RadioField(