New implementation of the webhooks

A lot of improvements in the webhooks appplication. It is now a "plug-in" application: if you run it, it will generate callback calls to hooks registered via an API; if you don't run it, nothing changes on BigBlueButton. It also doesn't require any changes in BigBlueButton's code for it to run, all information the application needs comes from redis.
Check the README.md for more information.

The old API calls on bbb-web used for webhooks are not necessary anymore, but were not removed from the code on this pull request.

The only change outside of labs/bbb-webhooks is the inclusion of the external meeting ID in the event `meeting_created_message`, see 8a89cd1a712efa00e573731cb337d614a402d0c5

Tests for it were only done manually in development. We'll be testing this more in the next weeks as we start using it for real. I should write some unit tests as well during this period.