Translation will be implemented in the future.
For now the "en" in /help/en/<pagename> is hardcoded.

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.

To facilitate translation of the user documentation we move
the templating logic outside of the user documentation.

Currently we have two kinds of user documentation:

* the about page[1] which is written in HTML and part of the web
  application and can therefore link instance-specific pages
  (like e.g. the preferences) via Jinja variables

* the Sphinx documentation[2] which is written in reStructuredText
  and cannot link instance-specific pages since it doesn't know
  which instance the user is using

The plan is to integrate the user documentation currently in Sphinx
into the application, so that it can also link instance specific pages.
We also want to enable the user documentation to be translated.

This commit implements the first step in this endeavor (see #722).

[1]: searx/templates/__common__/about.html
[2]: docs/user/ (currently served at https://docs.searxng.org/user/)