Allow setting of caseworking on a user

This commit changes the form that the user sees when inviting or editing
another user, if the service has the ‘caseworking’ permission set.

This will allow creating a new type of user, one who only has the
`send_messages` permission, without the `view_activity` permission.

We are doing this because we think there are a number of services with a
lot of users who don’t need to see the dashboard, or the other team
members, and that we can make a simpler interface for these users.

app/assets/javascripts/conditionalRadios.js

@@ -0,0 +1,25 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.ConditionalRadios = function() {
+
+    this.start = function(component) {
+
+      const $radios = $('[type=radio]', $(component)),
+            showHidePanels = function() {
+              $radios.each(function() {
+                $('#panel-' + $(this).attr('value'))
+                  .toggleClass(
+                    'js-hidden',
+                    !$(this).is(":checked")
+                  );
+              });
+            };
+
+      $radios.on('click', showHidePanels);
+      showHidePanels();
+
+    };
+  };
+
+})(window.GOVUK.Modules);