Add slack integration

[caarmen]

Post a message to a slack channel when a debt is created or deleted.

• Add an optional environment variable SLACK_WEBHOOK.
• Add signals for post_save and post_delete:
  • They are only invoked if SLACK_WEBHOOK is present (via a new decorator requiring an environment variable to call the wrapped function).
  • They construct a message and send it to the slack webhook.
• Format the message using a jinja2 template (jinja2 allows more control over whitespace than the django templating).
  • Add a dependency to jinja2.
  • Configure jinja2 in the settings.py and a jinja2.py module.
• Localize the text of the messages.
• In tests:
  • Mock the slack webhook from tests.
  • Test with multiple scenarios of slack being configured or not, or returning an error or not;
  • Assert that the slack webhook is called as expected.