Create block-label styles

* Base on the pattern in GOV.UK elements, but use the existing
Bootstrap palette, http://govuk-elements.herokuapp.com/#guide-forms
* Add examples to style guide
* Currently missing the javascript focus and selected styles
Use block labels in stats announcement forms

* Provide better titles and descriptions for statistics types, and
replace select with radio buttons for easier access to options
* Use inline block labels for display precision
Dynamically update example date precision

* Show users the exact date that will be published and shown to users
based on their choices. (As announcements are published immediately,
they otherwise wouldn’t see it until it was live)
* Exact date, one month range, two month range and whether the date is
provisional or not