Revisit logic of gamification and stats

We have to revisit and improve the logic about gamification. It would be nice to add some more stats about encryption etc. (e.g. see logging)

Steps:

• Brainstorming about stats and gamification
• Add test cases
• Do store our stats and gamification status?
• Add logic (see current version about gamification and see below)

We should check the calculation of badges:

• Invite a friend only when you send an invitation mail
• Number of secure mails
• Number of secure contacts
• GamificationData.getSecureProgress() results wrong data (mails are partially double counted).