Redesign of the mail functionality

[TamaroWalter]

This PR is a makeover of the mail functionality and accomplishes these things:

• Fix non-anonymous user information of an anonymous post, as mentioned in the notification mail about a new question / thread contains the link to the profile of the anonymous questioner #211 and When receiving a summary of forums I can see the name of a anonym questioner #202
• Improve the daily digest mail task as mentioned in Improve daily mail #208
• Restructure the mail management. Move functions from the lib.php to the mail_manager class. Make the mail_manager more readable and comprehensive.
• The new structure should contain different classes for each task: send subscription mails, review mails and daily digest mails.