Missing Sentry in Celery env

[webknjaz]

While working on Sentry integration for octomachinery, I was exploring how it's done in various places.
I came across #239 and it looks like it's only integrated into the web-server that receives events from GitHub but not into the environment that runs backporting tasks.

This https://docs.sentry.io/platforms/python/guides/celery/ suggests that the init invocation should also be added into the Celery worker runtime at the very beginning (best before any imports!).
It also suggests that sentry_sdk.integrations.celery.CeleryIntegration should be added explicitly.

cc @Mariatta FYI

It's probably a good first issue for some folks doing Hacktoberfest later in October.