Draft: Sync epics as work item

What does this MR do and why?

Outline of epic sync as a work item. It creates a work item of epic type for every normal epic created. Some key points:

1. It only syncs name and description
2. Adds a database column work_item_id to epic table to establish relationship. Later this column could be used to run background migrations to sync the whole epic table. It also holds unique indexes to ensure an epic can only have one work item assigned to it.
3. Uses a transaction to create epic and work item together. Still have to check if this is safe as it could be a long transaction that also executes some background jobs. If a transaction is not possible we could fake one like it was made for requirements sync.