is_verification_enabled flag added to models and admin (defaults to True); only content with this flag is verified

timeouts can be None, set to config.MAZ_REQUESTS_TIMEOUT in get_live_content method to make tests work

Values are configurable in constance live settings.

allows to send notifications at the end of the content_verify task

- both email and slack can be used at the same time;
- all SMTP configurations are tunable with constance
- slack configurations are tunable with constance
- recipients are in bcc
- from address is configurable in constance
live settings available for notification systems (email and slack)

docker image renamed and aligned between docker-compose.yml and .gitlab-ci.yml; uwsgi invocation corrected

constance requirements adjusted

taskmanager urls added

dynamic configuration with constance;
slack notification integrated

Feature/dockerizing stack

See merge request openpolis/op-sources-verification!1

the template has already been transformed into the one used in
dockerised stacks