a8n: Upsert ChangesetEvents from Bitbucket Server webhooks

Created by: mrnugget

This the Automation/Sourcegraph side of RFC 45 - Extend Bitbucket Server Plugin.

With the webhooks added in https://github.com/sourcegraph/bitbucket-server-plugin/pull/10 and their documentation added in https://github.com/sourcegraph/bitbucket-server-plugin/pull/11 we can add support for Bitbucket Server webhooks to Automation.

What we need to do is the Bitbucket Server equivalent of the GitHub webhook implementation: https://github.com/sourcegraph/sourcegraph/pull/5913