Convert diacritics to approximations before filtering

- Introduces a helper method for setting the filterable value
- Updates `filter_controller.js` to use `data-filter-text-value` attribute populated with converted content. This was previous `data-value` in the earlier implementation but ignored in the current on.
- Also uses the same helper to make @mentions diacritic insensitive

app/javascript/controllers/filter_controller.js

@@ -10,7 +10,7 @@ export default class extends Controller {
 
   filter() {
     this.itemTargets.forEach(item => {
-      if (item.innerText.toLowerCase().includes(this.inputTarget.value.toLowerCase())) {
+      if (item.dataset.filterTextValue.toLowerCase().includes(this.inputTarget.value.toLowerCase())) {
         item.removeAttribute("hidden")
       } else {
         item.toggleAttribute("hidden", true)