auth: Render config error page on configuration error.

We previously used to to redirect to config error page with
a different URL. This commit renders config error in the same
URL where configuration error is encountered. This way when
conifguration error is fixed the user can refresh to continue
normally or go back to login page from the link provided to
choose any other backend auth.

Also moved those URLs to dev_urls.py so that they can be easily
accessed to work on styling etc.

In tests, removed some of the asserts checking status code to be 200
as the function `assert_in_success_response` does that check.

templates/zerver/config_error.html

@@ -97,6 +97,7 @@
                     {% endif %}
                     <p>After making your changes, remember to restart
                     the Zulip server.</p>
+                    <p><a href=""> Refresh</a> to try again or <a href="/login/">click here</a> to go back to the login page.</p>
                 </div>
 
             </div>