Searchable Select: fix tab-back bug

make the shadow input non-tabable via tabindex=“-1” to prevent it from accepting tabs, fixes #1958

app/assets/javascripts/app/views/generic/searchable_select.jst.eco

@@ -6,6 +6,7 @@
     <%= @attribute.required %>
     <%= @attribute.autofocus %>
     value="<%= @attribute.value %>"
+    tabindex="-1"
   >
   <input
     class="searchableSelect-main form-control js-input<%= " #{ @attribute.class }" if @attribute.class %>"