[click packages] Add an option to reply to each review

An option to add a reply to a user-review and show them next to the review on the app page in "Ubuntu Store" will be a great feature. This will help developers/publishers to deal with harsh reviews due to not understanding the features of the app properly. Also it would help potential future users to not have a false impression (from a harsh review) about the app which they're going to try.

Bonus: Send the reply to the reviewer as an email.