Add Promotional features for executive offices

Executive offices can have any number of "promotional features". Each
feature is made up of one or more "promotional feature items", which
consists of an image, some text, an optional title (with link) and up
to six additional links. Promotional feature items can be flagged as
being "double-width". These will be displayed with a larger image when
shown on frontend. The best way to understand this is probably by
checking out the visuals attached to the tracker story:
https://www.pivotaltracker.com/story/show/46261745
Add links for promotional feature items

These are managed using the standard nested attributes mechanism in
rails. I've based the JavaScript on that used for managing contact
phone numbers. It is slightly different here in that it uses the
built-in mechanism for removing nested associations by setting the
_destroy param, rather than massaging them in the controller.
DRY up attachment-related code in tests
FormBuilder renders horizontal layout checkboxes
Wire up promotional features to frontend.

Displays the promotional features according to the number and size
of the items within them. I'm not entirely happy with the use of the
instance variable to keep track of the column number, but I couldn't
think of a cleaner way to do this, given that the width of the items
can vary. Also: in an ideal world, there would be separate partials
for the single and double-width promo items, and we'd set a
to_partial_path method on the model to return the appropriate partial
path, but that is only available in Rails >= 3.2.