member controller

[Integer93]

Closes #15

Created a controller to create members.

Changes

• department creation now also updates the admin, setting the department reference
• fixed problems with json serialization of circular dependencies
  • serialization goes 1 level deep and just prints the id instead of serializing another level
• created custom update query to clear a department reference from all members
  • is needed before removing the department, to avoid foreign key problems
• introduce spring-boot-starter-validation to validate incoming dto´s