CORE-5602 Implemented remaining Template AVU endpoints.

Implemented
GET /filesystem/:data-id/template-avus
DELETE /filesystem/:data-id/template-avus/:template-id
DELETE /filesystem/:data-id/template-avus/:template-id/:avu-id
CORE-5602 Refactored Metadata Template AVU endpoints.

Refactored database statements into functions in donkey.persistence.metadata.