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/assets/javascripts/radioSlider.js

@@ -0,0 +1,26 @@
+(function(global) {
+
+  "use strict";
+
+  global.GOVUK.Modules.RadioSlider = function() {
+
+    this.start = function(component) {
+
+      $(component)
+        .on('click', function() {
+
+          leftRight = $(this).find(':checked').next('label').text().split('/');
+
+          if (leftRight.length === 2) {
+            $(this).find('.radio-slider-left-value').html(leftRight[0]);
+            $(this).find('.radio-slider-right-value').html(leftRight[1]);
+          }
+
+        })
+        .trigger('click');
+
+    };
+
+  };
+
+})(window);