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
Change display precision to display date
Alter stats heading classes

* Avoid inferring semantics, the styles should be usable with any
content
* Don’t nest divs within headings, that’s not valid HTML
Rename admin template files as SASS partials

* Files aren’t intended to be compiled and used by themselves
* Use _ to indicate that they are partials —
http://sass-lang.com/guide#topic-4
Move stats CSS to helpers

* The stats CSS applies to a set of pages across a couple of
controllers, and isn’t restricted based on the usual controller
generated classnames
* Because it doesn’t meet the usual rules for styles specific to views,
it should live elsewhere