Massive JavaScript cleanup

- Use JSX for templates
- Docblock/comment everything
- Mostly passes ESLint (still some work to do)
- Lots of renaming, refactoring, etc.

CSS hasn't been updated yet.

js/lib/helpers/highlight.js

@@ -1,21 +1,37 @@
-import truncate from '../utils/truncate';
+import { truncate } from 'flarum/utils/string';
 
-export default function(string, phrase, length) {
-  if (!phrase) {
-    return string;
-  }
+/**
+ * The `highlight` helper searches for a word phrase in a string, and wraps
+ * matches with the <mark> tag.
+ *
+ * @param {String} string The string to highlight.
+ * @param {String|RegExp} phrase The word or words to highlight.
+ * @param {Integer} [length] The number of characters to truncate the string to.
+ *     The string will be truncated surrounding the first match.
+ * @return {Object}
+ */
+export default function highlight(string, phrase, length) {
+  if (!phrase && !length) return string;
 
+  // Convert the word phrase into a global regular expression (if it isn't
+  // already) so we can search the string for matched.
   const regexp = phrase instanceof RegExp ? phrase : new RegExp(phrase, 'gi');
 
-  let highlightedString = string;
+  let highlighted = string;
   let start = 0;
 
+  // If a length was given, the truncate the string surrounding the first match.
   if (length) {
-    start = Math.max(0, string.search(regexp) - length / 2);
-    highlightedString = truncate(highlightedString, length, start);
+    if (phrase) start = Math.max(0, string.search(regexp) - length / 2);
+
+    highlighted = truncate(highlighted, length, start);
   }
 
-  highlightedString = $('<div/>').text(highlightedString).html().replace(regexp, '<mark>$&</mark>');
+  // Convert the string into HTML entities, then highlight all matches with
+  // <mark> tags. Then we will return the result as a trusted HTML string.
+  highlighted = $('<div/>').text(highlighted).html();
 
-  return m.trust(highlightedString);
+  if (phrase) highlighted = highlighted.replace(regexp, '<mark>$&</mark>');
+
+  return m.trust(highlighted);
 }