Find documents to add to a series

We're using jQuery UI's autocomplete widget, but with its default
behaviour of searching the "source" for user input whenever there's a
short delay between key presses. Our server's current SQL based search
functionality can't return search terms fast enough to support it.

Instead, I've added a "Find" button next to the text input that users
must click to search for the text in the input field. Pressing [Enter]
also works.

The JavaScript for jQuery UI was already loaded in admin, so I just had
to get some missing CSS to get it working.