allow disabling editing a form in vellum

This is for forms that vellum breaks, so that we can block users from
mistakenly editing them in vellum.

This feature under is a toggle (/hq/flags/edit/no_vellum/),
and is basically intended to stay there indefinitely,
so that a Dimagi power-user can disable editing certain
forms and hide that option from others
(and their future selves if they wish).

The form can still be edited by choosing Advanced > Download,
editing by hand, and reuploading with Advanced > Upload.

The following modifications are made on form pages for which vellum is disallowed (whether or not the domain/viewing user has the toggle enabled):

![screen shot 2014-11-25 at 12 49 58 pm](https://cloud.githubusercontent.com/assets/137212/5183013/b93a8fc6-74a1-11e4-8161-e64769b92307.png)

Configurable in Advanced form settings, only if you have the toggle enabled for the domain or user:

![screen shot 2014-11-25 at 12 51 55 pm](https://cloud.githubusercontent.com/assets/137212/5183059/0c856fa2-74a2-11e4-9c2c-abc152296bae.png)