Fix some issues with the new file attachment display.

The new Update menu work for file attachments had a couple bugs that
prevented usage on new uploads, and broke tests.

Newly uploaded file attachments had an extra <li> tag, which broke the
tests and resulted in malformed HTML. They also didn't have a
functioning Update menu, since the menus were only ever created on
initial page render.

Existing file attachments had a stray "%}" in the class. This has
probably been there a while.

The menu could have been fixed with some more global menu registration
stuff, but it ended up getting messy. Instead, I've removed all the old
JS-level menu support, and simply moved it into CSS. Now, the browser
handles the display of menus for us. I also tweaked the animation time
to be a bit quicker and smoother.

To work around the issue of the Update menu sticking around on click
(which didn't work before, due to referencing a different class name,
and still didn't work here with that fixed), I've simply fixed the
z-orders so that the modal dialog overlay appears above the menu. The
menu is still on-screen until the mouse is moved away, but it's not
noticeable.

Reviewed at https://reviews.reviewboard.org/r/6445/