More fine tuning

[c8y3]

• endpoint GET /api/v2/cases, changed field cases into data, so that endpoints which return paginated lists are standardized
• endpoint GET /api/v2/cases/{identifier}/iocs, changed field iocs into data, so that endpoints which return paginated lists are standardized
• return 404 when trying to delete inexistant task
• return 404 when trying to delete inexistant IOC
• return 404 when trying to delete inexistant asset
• return 404 when trying to create an asset in an inexistant case
• moved code out of utils.py into the blueprints namespace (in access_controls.py and responses.py)
• some cleanups: removed some seemingly dead methods, marked some methods as private (underscore prefix), simple quotes vs. double quotes