brands: hardcode ISSUE_URL and some links on the about-page

A *brand* of searx is a fork which might have its own design and some special
functions which might bee reasonable in a special context.

In this sense, the fork might have its own documentation but not its own issue
tracker.  The *upstream* of a brand is always https://github.com/asciimoo from
where the brand-fork pulls the master branch regularly.  A fork which has its
own issue tracker is a spin-off and out of the scope of the searx project
itself.  The conclusion is:

- hard code ISSUE_URL (in the Makefile)
- always refer to DOCS_URL
- links in the about page refer to the *upstream* (searx project)
  except DOCS_URL
- "fork me on github" ribbons refer to the *upstream*

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

searx/templates/courgette/github_ribbon.html

@@ -1,3 +1,3 @@
-<a href="{{ brand.GIT_URL }}" class="github">
+<a href="https://github.com/asciimoo/searx" 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>