[refactor] improve the gorm transaction that covers flag snapshot in crud logic

[zhouzhuojie]

Some of the database logic is not wrapped into a transaction in crud.go, for example, https://github.com/checkr/flagr/blob/848bcf07b8ff4f6a4a056794d521c62535b2e3b8/pkg/handler/crud.go#L424

Expected Behavior

Ideally, all the database logic of a request can be wrapped into a transaction to avoid data corruption.

Current Behavior

Possible Solution

Use db.Transaction(func(tx *gorm.DB) error { https://gorm.io/docs/transactions.html#Transactions