[fix] make oscar theme more CSP compliant - col-checkbox

Add col-checkbox in::

    searx/static/themes/oscar/src/less/preferences.less

Replaced style in file::

    searx/templates/oscar/preferences.html

Reported-by: https://github.com/searxng/searxng/issues/57


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