Clarify delete button and hide it when not needed on microsite based on

feedback from Erin Weigel.

microsite/index.html

@@ -37,8 +37,8 @@ <h2>Sample Ratio Mismatch (SRM) Checker</h2>
                     <input v-model="item[1]" type="number" step="any" min="0" max="1" pattern="\d*" class="form-control" id="exp-{{ index }}" placeholder="Expected ratio"/> 
                   </div>
                   <div class="form-group col-xs-2 text-center">
-                    <label for="del-{{ index }}">Del #{{ index+1 }}</label>
-                    <button v-on:click="dropArm(index)" class="btn btn-default" type="button" role="button" aria-pressed="true">X</button>
+                    <label v-if="input.arms.length > 2" for="del-{{ index }}">Delete #{{ index+1 }}</label>
+                    <button v-if="input.arms.length > 2" v-on:click="dropArm(index)" class="btn btn-default" type="button" role="button" aria-pressed="true">X</button>
                   </div>
                 </div>
                 <div class="form-group text-center">