Add 500 page
Add 500 page

This should have been added along with the 404 template in 7c2d3c0 but
it wasn't. It's being added now. `xmas.frontend.create_app` sets up
error handlers for 404 and 500 errors, so there should have been
templates for both long before now.

dirn/Secret-Santa,dirn/Secret-Santa,dirn/Secret-Santa,dirn/Secret-Santa