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>