Build new endpoint to apply a template

Related to #875 (closed).

A new endpoint ApplyProcessTemplate must be exposed as POST /api/v1/dataimport/<template_id>, requesting for the user another Dataimport id in the body (that's tthe target).

That endpoint must check that the user has required contributor rights on both dataimports, and then:

• set the template field on the target
• copy all worker runs from the template onto the field
• raise a 403 if a template is already set on the target