Rename module

HighlightTags was bad because:
- we haven’t called placeholders ‘tags’ for a long time
- it also does resizing of the `<textarea>`, not just highlighting the
  placeholders

app/assets/javascripts/enhancedTextbox.js

@@ -7,7 +7,7 @@
 
   const tagPattern = /\(\(([^\)\((\?)]+)(\?\?)?([^\)\(]*)\)\)/g;
 
-  Modules.HighlightTags = function() {
+  Modules.EnhancedTextbox = function() {
 
     this.start = function(textarea) {