[TASK] Make search in file module case-insensitive

The JavaScript-Based search in the file module is case sensitive. This
is unusual for searches.

Both the searched value as well as the searchable data are now
transformed toLowerCase before comparison.

Possible improvements are:
 - Applying that transformation in Fluid, trading having it to do every
   request server-side vs. not having to do it on the client for every
   search.
 - Implementing some kind of fuzzy search-logic.

Resources/Public/JavaScript/BackendModule.js

@@ -167,12 +167,12 @@ define([
  const searchForm = document.querySelector('.js-form-search');
  if (searchForm) {
  new DebounceEvent('input', function (event) {
- const searchValue = event.target.value;
+ const searchValue = event.target.value.toLowerCase();
  const elements = document.querySelectorAll('.in2publish-stagelisting__item');
 
  (Array.from(elements)).forEach(function (item) {
  if (searchValue !== '') {
- const searchable = item.getAttribute('data-searchable');
+ const searchable = item.getAttribute('data-searchable').toLowerCase();
 
  if (!searchable.includes(searchValue)) {
  item.classList.add('d-none');