Mission Accomplishment Callback
Send e-mail to the user that complete the mission
- Enable the "I'm ready button" again (@fbezelga and @philss)
- Sends the complete mission e-mail
- Fix the "Next mission" link on congratulations page
- Improves the User#can_enroll? method(@fbezelga and @philss)

This commit is related to #137
Merge pull request #147 from playthecall/mission-accomplishment-callback

Mission Accomplishment Callback