Dates for administrative units
When refactoring I noticed that administrative units and historical places can't have associated date information like types.
Not sure why we didn't implement this when implementing for types (#1215). According to that 3 year old issue it was planned only for custom types but seems to be available for standard types now too.
Anyway, I implemented dates for administrative units and historical places in the develop branch too, because I think it useful.
But please let me know in case someone has arguments not to do this.