It was getting difficult to remember which of the normalizers
use None and which don't. So let's try to be a little more consistent
and just use empty sets, etc, so the caller can rely on receiving a
set instead of having to check for None.
Also renamed search parameter authors->author to be more in line
with the singular form of extension.
Adds CSS variables for the highlight and shadow so they don't need
to be custom-defined for each color of button.
Elements should use more generic classes like green_button instead of
add_tag_button unless there's a good reason.
Appearing on search and album pages, the tray is where you can
remove items from your clipboard without having to click on its
checkbox -- that photo card may not even be on the current page.
New frontends folder will hold all front-end interfaces for etiquette. Existing flask site moved here and refers to itself as a package with external launcher. etiquette_site renamed to etiquette_flask
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