Pull Request summary Cancel button needs confirmation

[StanleyGoldman]

The Cancel button noted here will remove all of a user's in progress review comments.
The user may not be aware of the effect and may simply be trying to leave the screen because they are not ready to complete their review.

The effect of the cancel button should be clearly explained with a dialog/confirmation before allowing the user to continue.

[meaghanlewis]

On dotcom, there is a confirmation for the Cancel review option explaining that all pending comments will be lost and this is really helpful.