Database reltaions

In the current branch `presentation-fixes` i "fixed" the retrieval of Petitions by changing `student_email` ---> `student_username` in the Petition model and also in the Employee model. In the course of this change I also changed the "get_employee_by_mail" function to
"get_employee_by_username". However we do not save the username in the Employee model. All these fixes slowly tend to become convoluted!

Do get this sorted we need to tackle this in a more structured way:

1. Make schematic of models.
2. Make Connections clear.
3. Find the best fields to indicate the relation between models.