Add data privacy policy to imprint

In Administration -> Customizing -> Imprint the imprint text and terms of use text can be added. By law, the data privacy policy must be a separate menu item, thus is needs to be added. In addition we make some other changes: 

Todo

• Add data privacy policy as third element
• Add option to turn on/off the contact form
• Add field for contact form email (already available as persisted properties, needs to read/write to existing place!)
• Modify run code to only show the imprint, terms of use and data privacy policy in the menu if there is actually something to be displayed. Currently is shows an error when no file is configured. 
• Implement fallbacks to an available language if the users language is not available.