Add profile settings (with optional name, e-mail, newsletter, stats)

[andrasfuchs]

The future of Freerouting should be influenced more by the users who use it the most. For this to happen it would be excellent if users could opt-in for allowing me to contact them directly. This would be strictly optional and e-mail addresses would be handled privately and securely.

For the implementation of this feature we need a new dialog that we would show after a high number of sessions were completed, also a mechanism to record the number of sessions completed per user.

Invitations would be limited to power users only (~top 0.5%) at the beginning.

Alongside this, it would be great to allow every user to join a Freerouting newsletter and give everyone an opportunity to share their success.