HINT: this patch has no functional change / it is the preparation for following
      changes and bugfixes

Over the years, the preferences template became an unmanageable beast.  To make
the source code more readable the monolith is splitted into elements.  The
splitting into elements also has the advantage that a new template can make use
of them.

The reversed checkbox is a quirk that is only used in the prefereces and must be
eliminated in the long term.  For this the macro 'checkbox_onoff_reversed' was
added to the preferences.html template.  The 'checkbox' macro is also a quirk of
the preferences.html we don't want to use in other templates (it is an
input-checkbox in a HTML form that was misused for status display).

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

Previously the preferences & stats templates contained the markup:

<a href="{{ url_for('index') }}"><h1><span>SearXNG</span></h1></a>

There are many things wrong with this:

1. the markup was duplicated

2. the CSS needed to be changed whenever a new page wanted to use this
   header (since the CSS used page-specific selectors)

3. h1 should be reserved for the actual page title
   (e.g. Preferences or Engine stats)

4. the image was set via CSS which also set:

       span { visibility: hidden; }

   which however removes the alternative text from the accessibility
   tree (meaning screen readers will ignore it).

This commit fixes all these problems.