preferences.py: implement defaults if unset

Implemnts defaults for:

- search.default_lang, search.autocomplete, search.safe_search,
- ui.default_theme, ui.default_locale
- server.image_proxy

This fixes also:

  https://github.com/asciimoo/searx/pull/1860#issuecomment-590082955



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