Update "col-xs" usage to bootstrap4

app/assets/javascripts/boards/components/modal/empty_state.js

@@ -41,10 +41,10 @@ gl.issueBoards.ModalEmptyState = Vue.extend({
   template: `
     <section class="empty-state">
       <div class="row">
-        <div class="col-xs-12 col-sm-6 order-sm-last">
+        <div class="col-12 col-md-6 order-md-last">
           <aside class="svg-content"><img :src="emptyStateSvg"/></aside>
         </div>
-        <div class="col-xs-12 col-sm-6 order-sm-first">
+        <div class="col-12 col-md-6 order-md-first">
           <div class="text-content">
             <h4>{{ contents.title }}</h4>
             <p v-html="contents.content"></p>

app/views/shared/dashboard/_no_filter_selected.html.haml

 .row.empty-state.text-center
-  .col-xs-12
+  .col-12
     .svg-130.prepend-top-default
       = image_tag 'illustrations/issue-dashboard_results-without-filter.svg'
-  .col-xs-12
+  .col-12
     .text-content
       %h4
         = _("Please select at least one filter to see results")

app/views/shared/empty_states/_wikis_layout.html.haml

 .row.empty-state
-  .col-xs-12
+  .col-12
     .svg-content
       = image_tag image_path
-  .col-xs-12
+  .col-12
     .text-content.text-center
       = yield