[fix] url_for(..., _external=True) in templates

The `url_for` function in the template context is not the one from Flask, it is
the one from `webapp`.  The `webapp.url_for_theme` is different from its
namesake of Flask and has it quirks, when called with argument `_external=True`.

The `webapp.url_for_theme` can't handle absolute URLs since it pokes a leading
'/', here is the snippet of the old code::

    url = url_for(endpoint, **values)
    if settings['server']['base_url']:
        if url.startswith('/'):
            url = url[1:]
        url = urljoin(settings['server']['base_url'], url)

Next drawback of (Flask's) `_external=True` is, that it will not return the HTTP
scheme when searx (the Flask app) listens on http and is proxied by a https
server.

To get the right scheme `HTTP_X_SCHEME` is needed by Flask (werkzeug).  Since
this is not provided in every environment (e.g. behind Apache mod_wsgi or the
HTTP header is not fully set for some other reasons) it is recommended to
get *script_name*, *server* and *scheme* from the configured `base_url`.  If
`base_url` is specified, then these values from are given preference over any
Flask's generics.

BTW this patch normalize to use `url_for` in the `opensearch.xml` and drop the
need of `host` and `urljoin` in template's context.

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