index.html: cluster menu

Reintroduce cluster by locus.
Partially revert 70bd7ef6.

browser/index.html

@@ -117,6 +117,7 @@
                     <a class="buttonSelector" onclick="m.resetClusters();">reset all user clones</a>
                     <a class="buttonSelector" onclick="m.clusterBy(function(id){return m.clone(id).getGene(5)});">cluster by V/5' </a>
                     <a class="buttonSelector" onclick="m.clusterBy(function(id){return m.clone(id).getGene(3)});">cluster by J/3' </a>
+                    <a class="buttonSelector" onclick="m.clusterBy(function(id){return m.clone(id).germline});">cluster by locus </a>
 
                     <div id="clusterby_button"> </div>
                 </div></div>