[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>