Adds verification to fourth grade checkbox.

Adds verification to fourth grade checkbox (please double-check my code!!!).  Closes 18F/ekip#136

ekip/everykid/templates/get-your-pass/student_pass.html

@@ -25,6 +25,19 @@ <h1>Start your adventure right now</h1>
       <button>Play</button>
     </form>
 
+  <form method="POST">
+      {% csrf_token %}
+
+      <label for="in_the_fourth_grade">
+      <input id="in_the_fourth_grade" required name="in_the_fourth_grade" type="checkbox" value="Y" onchange="this.setCustomValidity(validity.valueMissing ? 'Please indicate that you accept the Terms and Conditions' : '');" />
+      <span>Yes, I'm in the fourth grade or home-school equivalent</span></label>
+
+      <button>Play</button>
+    </form>
+
+<script type="text/javascript">document.getElementById("in_the_fourth_grade").setCustomValidity("First check the box if you are in the fourth grade.");</script>
+
+
   </div>
 
 {% endblock content %}