This includes a minor restructure of the autocomplete JS to make use of
promises and improve tracking of pending JS.

In particular it improves the way in which throttled text input is
handled to ensure that the behat does not continue until:
- typing is fully complete; and
- all possible ajax requests have been sent; and
- all possible ajax requests complete; and
- the suggestions are updated.

A number of conditions existed where behat would move on to the next
step too early in a race condition effect between Behat and Autocomplete.