replace border by box-shadox in flex-bloc and flex-table

We use box-shadow instead of border
to avoid to manage border double-width
when blocs are resized for small screen !

Then we can simulate border-collapse: collapse (table)

src/Bundle/ChillActivityBundle/Resources/public/vuejs/Activity/App.vue

@@ -84,16 +84,7 @@ export default {
 div.flex-bloc {
    margin-top: 1em;
    div.item-bloc {
-      flex-grow: 0; flex-shrink: 1; flex-basis: 25%;
-      
-      margin: 0;
-      border: 1px solid #000;
-      padding: 1em;
-      
-      &:not(:first-child) {
-         border-left: 0;
-      }
-      
+      flex-basis: 25%;
       ul.list-content {
          list-style-type: none;
          padding-left: 0;