Add a form to set priority of top 2 providers

Their priority should always add up to 100%. Currently we have to ensure
this by hand. Adding this form means there’s no way to not set their
combined priorities to 100%. And it’s a bit more of an intuitive UI than
two textboxes on separate pages.

app/main/forms.py

@@ -952,6 +952,22 @@ class ProviderForm(StripWhitespaceForm):
     priority = IntegerField('Priority', [validators.NumberRange(min=1, max=100, message="Must be between 1 and 100")])
 
 
+class ProviderRatioForm(StripWhitespaceForm):
+
+    ratio = RadioField(choices=[
+        (str(value), '{}% / {}%'.format(value, 100 - value))
+        for value in range(100, -10, -10)
+    ])
+
+    @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(
         'Type of contact details',

app/main/views/providers.py

@@ -7,7 +7,7 @@ from werkzeug.utils import redirect
 
 from app import format_date_numeric, provider_client
 from app.main import main
-from app.main.forms import ProviderForm
+from app.main.forms import ProviderForm, ProviderRatioForm
 from app.utils import user_is_platform_admin
 
 PROVIDER_PRIORITY_MEANING_SWITCHOVER = datetime(2019, 11, 29, 11, 0)
@@ -67,12 +67,19 @@ def edit_sms_provider_ratio():
         if provider['notification_type'] == 'sms'
     ], key=itemgetter('identifier'), reverse=True)
 
+    form = ProviderRatioForm(ratio=providers[0]['priority'])
+
+    if form.validate_on_submit():
+        provider_client.update_provider(providers[0]['id'], form.percentage_left)
+        provider_client.update_provider(providers[1]['id'], form.percentage_right)
+
     if len(providers) < 2:
         abort(400)
 
     return render_template(
         'views/providers/edit-sms-provider-ratio.html',
         versions=_chunk_versions_by_day(_get_versions(providers[0], providers[1])),
+        form=form,
     )