utils: resolve circular imports

[VMois]

Originated in #156 (comment)

suggestion: In my opinion, circular imports indicate that the code in modes.py and utils.py is too coupled and requires reorganization.

Possible idea of how to split the code:

• utils.py file with low-level functions that do not depend on anything from reana_db.models (e.g, build_workspace_path)

• quotas.py file with functions that are built on top of DB models and low-level functions (e.g, update_workflow_cpu_quota)

• move workflow_status_change_listener function from models.py to event_listeners.py

This will create a clear dependency path: utils.py => models.py => quotas.py => event_listeners.py