Webhooks: add support for Bitbucket

[thomascube]

There's already support for Bitbucket (Server) Pull Request generators but the webhooks sent by Bitbucket are not processed. They just end up with an response saying "Webhook processing failed: event not defined to be parsed".

In order to complete the support for Bitbucket as SCM provider, supporting their webhooks would be highly appreciated.

[manuelwallrapp]

We need the same feature as well.

[artemnbo]

+1

[gillesdiacon]

+1