[mod] in the preference page, show !bang of subgrouping categories

The names of the subgrouping categories in the preference page are translated,
to use this categories the user needs to know by which !bang the category can be
selected.  Related to "Make 'non tab category' bangs discoverable" in [#690].

Related:

- [#690] https://github.com/searxng/searxng/issues/690
- https://github.com/searxng/searxng/issues/1604
- https://github.com/searxng/searxng/pull/1545



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>