[enh] show categories not in any tab category in "Other" preferences tab

Previously we didn't have a good place to put search engines that don't
fit into any of the tab categories. This commit automatically puts
search engines that don't belong to any tab category in an "other"
category, that is only displayed in the user preferences (and not above
search results).

searx/webapp.py

@@ -59,6 +59,7 @@ from searx.settings_defaults import OUTPUT_FORMATS
 from searx.settings_loader import get_default_settings_path
 from searx.exceptions import SearxParameterException
 from searx.engines import (
+    OTHER_CATEGORY,
     DEFAULT_GROUP_NAME,
     categories,
     engines,
@@ -182,6 +183,7 @@ _category_names = (
     gettext('software wikis'),
     gettext('web'),
     gettext(DEFAULT_GROUP_NAME),
+    gettext(OTHER_CATEGORY),
 )
 
 _simple_style = (gettext('auto'), gettext('light'), gettext('dark'))
@@ -439,6 +441,7 @@ def render(template_name, override_theme=None, **kwargs):
     kwargs['theme'] = get_current_theme_name(override=override_theme)
     kwargs['categories_as_tabs'] = settings['categories_as_tabs']
     kwargs['categories'] = _get_enable_categories(categories.keys())
+    kwargs['OTHER_CATEGORY'] = OTHER_CATEGORY
 
     # i18n
     kwargs['language_codes'] = [l for l in languages if l[0] in settings['search']['languages']]