Use year independent dates for permission periods

[pdmnyberg]

Please describe the feature

As a bird ringing expert I want to define the period of permissions within a license as dates that are independent of the year for which the license is active so that the periods don't have to be updated each time the license is updated

Detailed description

• A permission should use a date specification that is independent of the year specification
• (Optional) Periods could possibly be named and reused between permissions in order to build in additional knowledge into the model while allowing for reuse of know facts.