Migration of represents to S3Represents

I have moved the code from represents to S3Represents, this was a task listed. Hopefully i have done it correctly as this is my first time contributing to sahana
This seems to just move the project_assignee_represent code into the constructor of the otherwise completely unrelated project_TaskRepresent class, but not to implement an S3Represent for the assignee key.

It is completely unclear how this is supposed to work - or how it would be called. As it is done here, it breaks both the representation of assignees and the representation of tasks. Not acceptable, I'm afraid.