This PR introduces the following changes:
- New Vue stores: `collaborationStore` and `pushConnectionStore`
- Front-end push connection handling overhaul: Keep only a singe
connection open and handle it from the new store
- Add user avatars in the editor header when there are multiple users
working on the same workflow
- Sending a heartbeat event to back-end service periodically to confirm
user is still active
- Back-end overhauls (authored by @tomi):
- Implementing a cleanup procedure that removes inactive users
- Refactoring collaboration service current implementation
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Co-authored-by: Tomi Turtiainen <10324676+tomi@users.noreply.github.com>
