Improve style customization of the configforms templates.

This enhances the configforms in a few ways to allow for more flexible
styling.

First, the boxes now make use of a "box-foot" class, which is used on
RBCommons. This extensions our box style convention to have a "head",
"main", and "foot", allowing for a separation of CSS rules for the main
part of a box and the footer.

It's also now possible to specify the base template name through a
base_template_name attribute on the ConfigPagesView.

Field rows will now contain a 'checkbox-row' class if it's a checkbox,
radio button, or a multi-select of checkboxes.

Finally, there was a typo where "clas=" was used instead of "class=",
which has been fixed.

Reviewed at https://reviews.reviewboard.org/r/5995/
Fix config forms item action and spinner styles in Firefox.

Firefox didn't display the item actions very well. They were being
wrapped to the following line, due to a combination of `float: right`
and `white-space: nowrap` (ironically).

To fix this, we now put all the action elements in a new <span> that's
floated right and placed as the first element in the item. This gives us
the look we want across all browsers.

The spinner is also now properly vertically aligned, and placed right
before any actions (if there are any).

Reviewed at https://reviews.reviewboard.org/r/5996/