[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>

searx/__init__.py

@@ -32,6 +32,27 @@ if max_request_timeout is None:
 else:
     logger.info('max_request_timeout=%i second(s)', max_request_timeout)
 
+_unset = object()
+
+def 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 :py:obj:`KeyError` is raised.
+
+    """
+    value = settings
+    for a in name.split('.'):
+        if isinstance(value, dict):
+            value = value.get(a, _unset)
+        else:
+            value = _unset
+
+        if value is _unset:
+            if default is _unset:
+                raise KeyError(name)
+            value = default
+            break
+
+    return value
 
 class _brand_namespace:  # pylint: disable=invalid-name