[mod] simple theme, JS: introduce window.searxng.theme namespace

In window.searxng.theme the theme data can be passed through to JS
implementations.

Initial the window.searxng.theme namespace starts with a value for
`img_load_error`::

    // image that is displayed if load of <img src='...'> failed
    img_load_error: 'img/img_load_error.svg'

The searx/static/themes/__common__/js/image_layout.js is patched to uses the
value, if the theme defines a value for img_load_error in this namespace.

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