Make Celery non-optional

We have the celry_optional which makes several things more complicated, because initially we wanted to support setups that do not run celery.

We should reconsider that in order to write code that always does what we expect it to do. In general, we are facing a tendency towards supporting one defined environment, and supporting that well.

Requirements:

• Make all Celery dependencies non-optional
• Remove celery_optional mechanisms
• Provide default way of waiting for a task
• Ensure deployment with Docker works
• Update documentation (Will be done in !333 (merged))
• Keep development environment simple (!468 (merged))