Email notifications v1.5

This PR introduces  an initial solution for email notification.  The whole issue is here: #1004 
What this PR contains:
- For every reply the owner (if not the same as the reply-creator) gets an email notification
- We have a pt schema for this mail.
- The user db schema is extended to support #581 
- The backend is ready for #581 
- New operators were added the streaming for checking a length of a field.
- We use this solution for sending out the reply notifications.
- Our e-mail notification system is extendable, so it is very easy to introduce new e-mail notification types