Get rid of custom styles for horizontal forms.

Forms in Cartridge now follow the same layout as Mezzanine (label
on top instead of label on the side).
Float buttons left and right in all forms throughout Cartridge.

This assumes the clearfix hack has been applied to .form-actions in
mezzanine.css.
Insert the "Order History" button by extending a {% block %}.

We can now get rid of the forced CSS positioning. This assumes the
"account_form_actions" block has been added to account_form.html in
Mezzanine.
Assign a class to the user panel for styling.

This assumes a CSS rule has been added for .user-panel in mezzanine.css.
Use {% error_for %} to render form errors.
Display card select and card expiry fields under their labels.

For consistency with the rest of the form.
Merge pull request #2 from jerivas/bootstrap3

Last tweaks for BS3 migration