Make promise mock work more like promise

This fixes the currently broken test. Promises are chainable and
previously this wasn't chainable. This makes the methods chainable.
Merge pull request #589 from alphagov/fix-tests

Make promise mock work more like promise
UI updates to unified search

- Update govuk_frontend_toolkit for measurements file
- Increase height of filter lists so you can see more options by default
- Comma separate large numbers for GOV.UK style
- remove 'on GOV.UK' as users found it confusing
- use 'remove filters' as it is clearer as to how it works
- Make filter description more generic