[filtron.sh] make filtron rule file configurable

To select a different file with filtron rules, set environment

    FILTRON_RULES_TEMPLATE

the default is

    utils/templates/etc/filtron/rules.json

The installation is done by the new function install_rules() which offers a
multiple choice in case of collisions (known from searx.sh install setup).

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