Review-mode

[michael]

First step is introducing comments. That way we can combine create and review stage. We may later want to think about a 'review' mode for the writer where just review tools are shown and contenteditable is set to false.

Also:

• how to express that a reference is not suitable as a reviewer. (you could put a comment next to the text that has the reference but maybe that's not explicit enough.)
• should reviews have some editing capabilities (that are presented as patches) or just be able to mark things as wrong or make suggestions?
• should there be the concept of issues that need to be resolved (or can we just treat comments as such)