Fix/3.0.0

This PR resolves the following issues:
- long load time of messages with large attachments
- unescaped sub-delimiter in attachment ids lead to 404
- fixed broken folder navigation
- error while upload attachment when message hasn't an id
- paginated mail overview should be store in browser history
- decode html entities in plain text
- servlet spec 3.0 required due to problems with loading files from webjars in Tomcat 6.0.x