[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/settings_defaults.py

@@ -201,6 +201,7 @@ SCHEMA = {
         'infinite_scroll': SettingsValue(bool, False),
         'cache_url': SettingsValue(str, 'https://web.archive.org/web/'),
         'search_on_category_select': SettingsValue(bool, True),
+        'hotkeys': SettingsValue(('default', 'vim'), 'default'),
     },
     'preferences': {
         'lock': SettingsValue(list, []),