Track the terms people put in the filter lists

There are two reasons to want to do this. Firstly is to see if anyone
uses them. The second is to see what people are using them for and what
kinds of things they think they can put in them.

I have put the tracking behind a throttler so we don't track every
keypress but just the word or words when they have finished typing.
Remove world transition bar

It is no longer needed. This is bassically a revert of 4e4e1eb.

https://www.pivotaltracker.com/story/show/47295571
Merge pull request #418 from alphagov/track-filters

Track the terms people put in the filter lists
Merge pull request #419 from alphagov/remove-world-bar

Remove world transition bar

https://www.pivotaltracker.com/story/show/47295571