Add github reporter status-url config option

Make it possible to set the github status link to something other than the
zuul server status_url. This allows the github status to link to the log output
for the jobs. If set, this url will be run through the same formatter
as the url_pattern used for comment links. If unset, it will fall back to the
previous behaviour of using the zuul server status_url.

Handle status-url from the reporter config and the zuul config the same
way and remove the hard coded change url format from zuul.conf. If a
user wants to specify the change id in the status url they should use
a template string in the same way as configuring a reporter.

Co-Authored-By: Jamie Lennox <jamielennox@gmail.com>
Change-Id: I5f2ff35bb38426ddcd7f65798b4f114256a54847
diff --git a/doc/source/reporters.rst b/doc/source/reporters.rst
index e3ab947..dd053fa 100644
--- a/doc/source/reporters.rst
+++ b/doc/source/reporters.rst
@@ -44,6 +44,10 @@
   set as the commit status on github.
   ``status: 'success'``
 
+  **status-url**
+  String value for a link url to set in the github status. Defaults to the zuul
+  server status_url, or the empty string if that is unset.
+
   **comment**
   Boolean value (``true`` or ``false``) that determines if the reporter should
   add a comment to the pipeline status to the github pull request. Defaults