Give parts a darker background

* Publishers use the white on grey contrast to identify fields they
need to edit, the white background of the default panel hides the form
fields from users scanning the page
* Give the panel a darker background, and make the title of the panel
darker still
Improve vertical alignment of Parts headers

* Position centrally in the space available
Tweak chevron position in expandable parts

* Nudge chevron within panel heading down slightly for better alignment
with text
Merge pull request #214 from alphagov/parts-contrast

Improve contrast of editable fields within "parts"