[#829] Add validator for Source question

This special rule checks that each Source URL is a valid URL for the
location quesiton which uses `QuestionFieldSource` question type.

census/ui_app/validators.js

@@ -12,6 +12,17 @@ let validators = {
       return !validator.isEmpty(value.toString());
     },
     message: 'Question is required'
+  },
+  sourceIsURL: {
+    rule: value => {
+      for (let i = 0; i < value.length; i++) {
+        if (!validator.isURL(value[i].urlValue)) {
+          return false;
+        }
+      }
+      return true;
+    },
+    message: 'Each source URL must be a valid URL'
   }
 };