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.
Instead of applying the background-color to the holder, apply it to the editor
area and message area separately. Then they can flex differently so that
the editor always maintains full height, and the message area shrinks to
a minimum of 30px.
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
