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.
