feat: use unread icon in mobile

templates/partials/category-selector-content.tpl

 <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">
     <span component="category-selector-selected">{{{ if selectedCategory }}}<span class="fa-stack" style="{function.generateCategoryBackground, selectedCategory}"><i class="fa fa-fw fa-stack-1x {selectedCategory.icon}" style="color: {selectedCategory.color};"></i></span> {selectedCategory.name}{{{ else }}}
-    {{{ if selectCategoryLabel }}}{selectCategoryLabel}{{{ else }}}[[topic:thread_tools.select_category]]{{{ end }}}{{{ end }}}</span> <span class="caret"></span>
+    <span class="visible-sm-inline visible-md-inline visible-lg-inline">{{{ if selectCategoryLabel }}}{selectCategoryLabel}{{{ else }}}[[topic:thread_tools.select_category]{{{ end }}}</span><span class="visible-xs-inline"><i class="fa fa-fw {{{ if selectCategoryIcon }}}{selectCategoryIcon}{{{ else }}}fa-list{{{ end }}}"></i></span>
+    {{{ end }}}</span> <span class="caret"></span>
 </button>
 <div component="category-selector-search" class="hidden">
     <input type="text" class="form-control" autocomplete="off">