Improve styling and extensibility for the My Account page.

The My Account page's styling was less than optimal when trying to put
<p> tags on the form. They were too large, had bad margins, and the
black text stood out too much. Now <p> tags have the same size and color
as labels, and have a good margin between elements, helping them to
blend in and feel natural.

Checkboxes also now have a sane style when not using the "checkbox-row"
class. This is particularly handy for MultipleChoiceFields. Previously,
the checkbox portion was taking 21em, which made no sense.

There's also some better extensibility for custom templates for the
page. Templates can inherit from prefs_page_form.html and overrie the
"pre_fields" and "post_fields" blocks to inject custom HTML before or
after the fields. Useful for explanatory text.