Workaround for firefox flexbox bug

Fixes selection while clicking on empty flexbox space.

This only happens in firefox and seems to be a bug in firefox
flexbox implementation, where the first child element with a
non zero size, in a flexbox managed `block` element, has the
`float` property.

Fixes #405

webadmin/src/views/apps.html

@@ -362,6 +362,9 @@
 
 <div class="content content-large">
 
+    <!-- Workaround for select-all issue, see commit message -->
+    <div style="font-size: 1px;">&nbsp;</div>
+
     <div class="animateMeOpacity ng-hide" ng-show="installedApps.length === 0 && user.admin">
         <div class="col-md-12" style="text-align: center;">
             <br/><br/><br/><br/>