[mod] add hotkeys option to settings.yml

The change in the hotkey mechanism introduced in 317db5b04 does not allow
configuration via `settings.yml`.  This commit adds that functionality.

Closes: #2898

searx/preferences.py

@@ -466,7 +466,7 @@ class Preferences:
                 locked=is_locked('search_on_category_select')
             ),
             'hotkeys': EnumStringSetting(
-                'default',
+                settings['ui']['hotkeys'],
                 choices=['default', 'vim']
             ),
             # fmt: on