[mod] infinite_scroll as preference

* oscar theme: code from searx/plugins/infinite_scroll.py
* simple theme: new implementation

Co-authored-by: Markus Heiser <markus.heiser@darmarIT.de>

searx/settings.yml

@@ -169,7 +169,6 @@ outgoing:
 #   - 'Ahmia blacklist'  # activation depends on outgoing.using_tor_proxy
 #   # these plugins are disabled if nothing is configured ..
 #   - 'Hostname replace'  # see hostname_replace configuration below
-#   - 'Infinite scroll'
 #   - 'Open Access DOI rewrite'
 #   - 'Vim-like hotkeys'