Create persistent model for user_competency

This issue is about creating the basic API create CRUD user competencies.

User competencies are links between the user and a competency, often they will be transparently created from a user plan.

This issue should not be creating any external function or API yet as we will not even have a view, and also because the capability checks are tricky in this case as the model is only used to store information at this stage. Later on when we add competency reviews then we will have API and external functions.

The persistent model is the only thing we need.

Non-exhaustive list of columns needed:

• int id
• int userid
• int competencyid
• int status (idle, in review, etc, will be defined later) (use constants)
• int reviewerid
• bool proficiency
• int grade (from a scale)

At this stage we will consider that the linked competency cannot be deleted without deleting the user_competency, therefore we do not need to duplicate the competency information like scale, etc...