Integration of Message Module

Problem/Motivation

Users should receive messages, when other community members edit/comment/rate his/her entities.

Proposed resolution

The Message module should be integrated.

User interface changes

Messages should be displayed in a drop-down in header on right side with red bubble for number of new messages.

API changes

For now, an email should be sent with every message.
Later on the messages should optional be collected and sent once a day (or optional a week).