Move the code coverage ingestion in mozilla-central

[marco-c]

Instead of running the bot off of a Taskcluster hook that is triggered by pulse or by the code-coverage-events service, we could make the task run in mozilla-central pushes directly as part of the m-c taskgraph (either by moving the code coverage bot directly in mozilla-central, or by using it as an external project).

This reduces the amount of code and services we have to maintain:

• we can get rid of the code-coverage-events service completely;
• we can get rid of the Taskcluster hooks to trigger coverage ingestion.