[mod] drop usage of the searx.brand namespace (python procs)

Added function searx.get_setting(name, default=_unset):
  Returns the value to which ``name`` point.  If there is no such name in the
  settings and the ``default`` is unset, a KeyError exception is raised.

In all the python processes ..

- make docs
- make buildenv
- make install (setup.py)

the usage of the 'brand.*' name space is replaced by 'searx.get_setting'
function.

- brand.SEARX_URL        --> get_setting('server.base_url')
- brand.GIT_URL          --> get_setting('brand.git_url')
- brand.GIT_BRANCH'      --> get_setting('server.base_url')
- brand.ISSUE_URL        --> get_setting('brand.issue_url')
- brand.DOCS_URL         --> get_setting('brand.docs_url')
- brand.PUBLIC_INSTANCES --> get_setting('brand.public_instances')
- brand.CONTACT_URL      --> get_setting('general.contact_url', '')
- brand.WIKI_URL         --> get_setting('brand.wiki_url')
- brand.TWITTER_URL      --> get_setting('brand.twitter_url', '')

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

setup.py

@@ -8,7 +8,7 @@ import os
 import sys
 
 from searx.version import VERSION_STRING
-from searx import brand
+from searx import get_setting
 
 with open('README.rst', encoding='utf-8') as f:
     long_description = f.read()
@@ -24,10 +24,10 @@ setup(
     version=VERSION_STRING,
     description="A privacy-respecting, hackable metasearch engine",
     long_description=long_description,
-    url=brand.DOCS_URL,
+    url=get_setting('brand.docs_url'),
     project_urls={
-        "Code": brand.GIT_URL,
-        "Issue tracker": brand.ISSUE_URL
+        "Code": get_setting('brand.git_url'),
+        "Issue tracker": get_setting('brand.issue_url')
     },
     classifiers=[
         "Development Status :: 4 - Beta",