Refactor groups API logic into GroupsManager

[davelopez]

Overview

More work to ease the transition to FastAPI.

• Added API tests for all endpoints
• Replaced the old legacy_expose_api decorators.
• Moved the controller logic to a manager class.
• Refactored the manager to handle errors raising the appropriate exception.

Additional comments

• I removed some of the log.info entries that looked like debug leftovers, but if they are important I can put them back :)
• The update action was not checking that updating the name of the group could conflict with an existing name so I added the check.