Release config ships dev_mode, use_stacktrace and debug logging

[Taure]

config/sys.config is what relx bakes into the image and it carries {dev_mode, true}, {use_stacktrace, true} and {logger_level, debug}. Together with logger:debug("message: ~p~n", [Message]) in ldf_receiver_controller the shipped container logs every intercepted message body in full and returns Erlang stack traces to HTTP clients on a 500. Wrong default for a lawful-interception demo; split a prod config or flip the defaults.

Surfaced by the review of #13.