[simple] introduce page_with_header.html template

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.