[mod] drop usage of the searx.brand namespace (templates & /config)

In the templates and the /config (JSON) the usage of the 'brand.*' name
space is replaced by 'searx.get_setting' function.

- new_issue_url          --> get_setting('brand.new_issue_url')
- brand.GIT_URL          --> get_setting('brand.git_url')
- brand.PUBLIC_INSTANCES --> get_setting('brand.public_instances')
- brand.DOCS_URL         --> get_setting('brand.docs_url')
- brand.ISSUE_URL        --> get_setting('brand.issue_url')
- brand.CONTACT_URL      --> get_setting('general.contact_url', '')

The macro 'new_issue' from searx/templates/*/messages/no_results.html
is now imported with context::

    {% from '__common__/new_issue.html' import new_issue with context %}

To get *public instances URL* from context's 'get_setting()' function::

    get_setting('brand.public_instances','')

Macro's prototype does no longer need the 'new_issue_url' argument and has been
changed to::

    macro new_issue(engine_name, engine_reliability)

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

searx/templates/simple/messages/no_results.html

@@ -11,7 +11,7 @@
   </p>
   {%- endfor %}
 
-  <p><small>{{ _('Please, try again later or find another searx instance.') }} (<a href="{{ brand.PUBLIC_INSTANCES }}">{{ _('Public instances') }}</a>) </small></p>
+  <p><small>{{ _('Please, try again later or find another searx instance.') }} (<a href="{{ get_setting('brand.public_instances') }}">{{ _('Public instances') }}</a>) </small></p>
 </div>
 {% else %}
 <div class="dialog-error" role="alert">