autocomplete: lowercase searches clientside.

Lowercase searches clientside before sending them to the server. This is
to improve cache hit rates when users accidentally type in uppercase,
especially on mobile where the first letter often gets capitalized.

app/javascript/src/javascripts/autocomplete.js

@@ -97,7 +97,7 @@ Autocomplete.current_term = function($input) {
   let query = $input.get(0).value;
   let caret = $input.get(0).selectionStart;
   let regexp = new RegExp(`^[-~(]*(${Autocomplete.tag_prefixes().join("|")})?`);
-  let match = query.substring(0, caret).match(/\S*$/)[0].replace(regexp, "");
+  let match = query.substring(0, caret).match(/\S*$/)[0].replace(regexp, "").toLowerCase();
   return match;
 };