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.
Add documents to a group in a series
Create empty group when creating a doc series
Show loading indicator while searching for docs
Remember which doc series group last document was added to
Update doc series cucumber scenarios
Remove documentFinder and tests
Don't compile mustache when running cucumber

We needed to do this while there was cucumber coverage of code that uses
mustache, to ensure that the latest templates were available to the
cucumber scenarios.

I've just modified the admin UI that used cucumber (the doc code that
finds docs when adding them to a doc series), so we no longer need this
step. I think we're better off without it as (should cucumber not
finish) compiled templates will be left on disk, and they may
accidentally get committed.