Add validation to challenge completion

Change ajax requests to send and accept JSON
to preserve data types.
Fix typos

server/middlewares/validator.js

@@ -1,9 +1,17 @@
 import validator from 'express-validator';
 
-export default validator.bind(validator, {
-  customValidators: {
-    matchRegex: function matchRegex(param, regex) {
-      return regex.test(param);
+export default function() {
+  return validator({
+    customValidators: {
+      matchRegex(param, regex) {
+        return regex.test(param);
+      },
+      isString(value) {
+        return typeof value === 'string';
+      },
+      isNumber(value) {
+        return typeof value === 'number';
+      }
     }
-  }
-});
+  });
+}