Fix "Sources::filter"

The "showNsfw" filter could only filter for either all sources or sources that aren't nsfw.
The "pinned" and "enabled" filtered behaved similarly, but at the same time they were also completely buggy.
E.g. "pinned = false" just returned all sources including pinned ones.

src/features/global-search/screens/SearchAll.tsx

@@ -259,7 +259,7 @@ export const SearchAll: React.FC = () => {
     const filteredSources = useMemo(
         () =>
             Sources.filter(sources, {
-                showNsfw,
+                isNsfw: showNsfw ? undefined : false,
                 languages: shownLangs,
                 keepLocalSource: true,
                 pinned: shouldShowOnlyPinnedSources,