adding select all none buttons

rowers/templates/mapcompare.html

@@ -20,7 +20,7 @@
 <h1>Course Map Comparison</h1>
 
 <p>
-  <form method="GET">
+  <form method="GET" id="competitorform">
     {% for competitor in competitors %}
     {% if competitor in selected_users %}
     <input type="checkbox" name="users" value="{{ competitor.id|encode }}" checked>{{ competitor }}<br>
@@ -28,8 +28,24 @@
     <input type="checkbox" name="users" value="{{ competitor.id|encode }}">{{ competitor }}<br>
     {% endif %}
     {% endfor %}
+    <button type="button" onclick="selectAll()">Select All</button>
+    <button type="button" onclick="selectNone()">Select None</button>
     <input type="submit" value="Submit">
   </form>
+  <script>
+    function selectAll() {
+      var checkboxes = document.getElementsByName('users');
+      for (var i=0; i<checkboxes.length; i++) {
+	checkboxes[i].checked = true;
+      }
+    }
+    function selectNone() {
+      var checkboxes = document.getElementsByName('users');
+	for (var i=0; i<checkboxes.length; i++) {
+	    checkboxes[i].checked = false;
+	}
+    }
+  </script>
 </p>
 <p>Click on a line to see the label. Double click on a line to remove it. Reload to get back all lines.</p>
 <p>We show your original GPS coordinates on the course. For course time, we use a finer interpolation, to get