classes are now uppercase and in their own files. lowercase classes are deprecated.
Interfaces are now actual interfaces that should be implemented instead of creating and returning an ai Object every time
(cherry picked from commit 5e3c67a5cf)
- date_helper, which was an et2_date_widget, now just a function that can handle all the different date formats in calendar
- holiday fetch & cache moved into its own file, useable from anywhere
