Add form CSS and examples to Style Guide

See: https://bugzilla.mozilla.org/show_bug.cgi?id=859427

Building out the form CSS for bedrock and adding corresponding examples to the Style Guide. So far, includes better error styles, fieldset/legend/label styles, and easier checkbox/radio-button lists.

Also standardized on ```.field``` rather than ```.form-field``` CSS class name, which was used in some places.

Updating the forms used across the site to take advantage of this new common code can follow in a subsequent pull request.