Updated some comment elements and standardised more JS

- Updated comment routes to be simpler.
- Updated comments JS to align better with updated component system.
- Documented available global JS functions/services.
- Removed redundant controller method.
- Added window.$events helpers for validation messages and
success/error.
- Updated JS events system to not be class based for simplicity.
- Added window.trans_plural method to handle pluralisation/replacements
where you already have the translation string itself.

Fixes #1836

resources/js/services/http.js

@@ -69,7 +69,10 @@ async function dataRequest(method, url, data = null) {
 
     // Send data as JSON if a plain object
     if (typeof data === 'object' && !(data instanceof FormData)) {
-        options.headers = {'Content-Type': 'application/json'};
+        options.headers = {
+            'Content-Type': 'application/json',
+            'X-Requested-With': 'XMLHttpRequest',
+        };
         options.body = JSON.stringify(data);
     }