auto generate a dropdown year nav

[dcnb]

No description provided.

[evanwill]

added in 3b312f5

note: when auto generating the years for the drop down, you set an increment in the theme.yml, then it tests each unique year by dividing it--so in theory, with a weird distribution of years, a collection could end up with not enough years in the drop down, e.g. you set 5 as the increment, but somehow the collection has no items with years divisible by 5...
so then, user should just manually set the years...