[TASK] Add Bootstrap fallback for missing -xs- class

Resources/Private/Backend/Partials/Filter.html

 {namespace be=TYPO3\CMS\Backend\ViewHelpers}
 <f:form action="index" controller="Backend" method="post" objectName="filters" object="{filters}">
 	<div class="row">
-		<div class="col-xs-6">
+		<div class="col-xs-6 col-sm-6">
 			<div class="form-group">
 				<label for="filter-locations">
 					<f:translate key="backend.filters.locations" />
@@ -20,7 +20,7 @@
 				</f:for>
 			</div>
 		</div>
-		<div class="col-xs-6">
+		<div class="col-xs-6 col-sm-6">
 			<div class="form-group">
 				<label for="filter-search"><f:translate key="backend.filters.search" /></label>
 				<f:form.textfield class="reset-me form-control" property="search" id="filter-search" />