The page_header macro includes an optional back link. Since the
page_header is always used inside `<main>`, where the back link should
not be, this stops setting the back link in the page header and instead
sets it in the new `backLink` block.
Because we won’t be showing uploaded letters individually on the uploads
page any more we need a way of listing them. This should be by printing
day, to match how we’re grouping them on the uploads page.
This code reuses the notifications.html template, but flips the
precedence of the filename and recipient because I reckon when you’re
looking at uploads you’re thinking filename-first.
