[fix] brands: add variables from build env to jinja templating

We have some variables in the build environment which are also needed in the
templating process.  Theses variables are relavant if one creates a fork with
its own branding.  We treat these variables under the term 'brands'.

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

searx/templates/courgette/github_ribbon.html

@@ -1,3 +1,3 @@
-<a href="https://github.com/asciimoo/searx" class="github">
+<a href="{{ brand.GIT_URL }}" class="github">
     <img style="position: absolute; top: 0; right: 0; border: 0;" src="{{ url_for('static', filename='img/github_ribbon.png') }}" alt="Fork me on GitHub"  class="github"/>
-</a>
+</a>