Fix #4174: Don't allow invalid prefix combinations for autocomplete.

* Autocorrect duplicated operators (e.g. complete `--tag` as `-tag`).
* Don't accept invalid metatag + prefix combinations (e.g. don't
  complete `char:rating:s` as `rating:safe`).

test/application_system_test_case.rb

@@ -2,5 +2,5 @@ require "test_helper"
 
 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   include SystemTestHelper
-  driven_by :selenium, using: :firefox, screen_size: [1400, 1400]
+  driven_by :selenium, using: :headless_firefox, screen_size: [1400, 1400]
 end