Last tweaks for BS3 migration

These commits iron out the last few kinks found in #001:

- Simplified form styles.
- Form action buttons correctly aligned to the left or right.
- Order History button is no longer shoe-horned into the Account edit template.
- User panel buttons are given more breathing room.
- Form errors are no longer repeated. They now appear consistently at the top of forms.

Depends on stephenmcd/mezzanine#848