Document Editor: Week 3

This week I worked on revision implementation. As proposed by Lennard I tried to refactor soc.models.work.Work, but found out that Work is also the base class for Quiz, Proposal, Documentation, Question and Survey. Daniel also proposed to implement revisions as three different models. That's why I decided to leave Work for models that doesn't support revisions and implement separate revision-related models.

In a couple of days I plan to implement logic for revisable models, refactor create function and implement view to list all the revisions.