Fix createdAt date range filter

- hiddenName has been disabled in order to fix the missing date range
  values when using the material theme
- data format has been changed to 'Y-m-d' in order to comply with the
  browser date input default format
- tests: date() and strtotime have been replaced with DateTime-related
  objects

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

app/Resources/static/themes/material/index.js

@@ -57,9 +57,9 @@ $(document).ready(() => {
   $('.datepicker').pickadate({
     selectMonths: true,
     selectYears: 15,
-    formatSubmit: 'dd/mm/yyyy',
-    hiddenName: true,
-    format: 'dd/mm/yyyy',
+    formatSubmit: 'yyyy-mm-dd',
+    hiddenName: false,
+    format: 'yyyy-mm-dd',
     container: 'body',
   });