Send an activation email for users

[sebbel]

Currently people can register with any email they prefer. This would leave some space for erroneus emails, where users cannot recover their account, because they entered a false email.

Please implement a mailing system to ensure that people are the owner of emails they are registering with.

As a user, I would like to have a safety-net which prohibits me from entering a false email, that I don't own.

As a project owner, I would like people to sign up with real existing emails.

NOTE: This can be achieved with mailgun, but needs to be documented in the readme for forkers.