FlywayPlugin: Prompt the user *again* before applying migration

[kmanning]

• If a pending migration is detected, when the user hits apply, prompt the user again, with a message, "There is a pending migration - are you sure you want to apply it?"
• Migrations will likely be an infrequent event, so this is a way to optionally draw extra attention to a developer, to make them aware that they're making a change that they might not realize.
• If this feature is not desired, the user can optionally disable it.