Make tag autocomplete suggestions case-insensitive

Since tags are all lowercase, no suggestions would be found if the user
typed in any uppercase letters. This converts to lowercase and fixes
that issue.

tildes/static/js/behaviors/autocomplete-input.js

@@ -125,7 +125,7 @@ $.onmount('[data-js-autocomplete-input]', function() {
         }
         var $tagsHiddenInput = $('[data-js-autocomplete-hidden-input]');
         var suggestions = $this.data('js-autocomplete-input').filter(function(suggestion) {
-            return suggestion.startsWith($this.val()) &&
+            return suggestion.startsWith($this.val().toLowerCase()) &&
                 !$tagsHiddenInput.val().match(new RegExp('(^|,)' + suggestion + ','));
         });
         if (suggestions.length === 0) {