www/py-horizon: Update to 17.1.0

• Also assign the port to Django 2.2 because Django 1.11 is End-of-Life since April.

• Do the same for its dependencies and bump PORTREVISION accordingly.

• Add an option to let users choose between the supported caching backends.
  
  Although databases/py-python-memcached isn't listed in requirements.txt it's used as default caching backend and needed at runtime.

PR: 245432
Approved by: maintainer timeout (14 days)