Update DB scheme

add unique key to User.Name and change the default value of User.Suspended to 1.
Fix broken createUser function
Implement getSession function
Refactor login method

to throw an exception if a suspended user tries to log in.
Fix broken getSession function
Refactor login method

to return if a particular user is an administrator.
Add methods to get if the user is an administrator, if the user is
superior to another user, to promote a user
Refactor user activation
to return the affected user's id.
Remove comment
since it obviously is deprecated.
Add a method to get if an administrator is deletable, change promoteUser
to changeLevel and fix some bugs