add sensible defaults to `Custom Message` Cognito trigger prompts

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when there is no default for common questions like,

> Enter the subject for your custom account confirmation email

**Describe the solution you'd like**
I think a default should exist so developers have something they can respond to.

> Enter the subject for your custom account confirmation email: confirm your account

**Describe alternatives you've considered**
I entered the phrase:
confirm your account