Qualification tracking

Add a feature for tracking qualifications.

A Qualification is something a person can be certified for. It has:

A name
A description
An (optional) responsible group (responsible for verifying, or helping gain, this qualification)
An (optional) set of skills that can be certified by this qualification

A Certification links a Qualification to a person. It has:

A relation to a Qualification
A relation to a Person who is certified
A date of validity
An (optional) date of expiry
A relation to a Person who verified the certification
An (optional) comment

The feature should exist under its own menu entry somehow. When this app was created, it was suppoed to only track skills and pedagogic goals, which is now extended (we will rename the app later on). If this issue results in restructuring the menu, please do.