Use predictable column names for content

[wioux]

ContentHelper#preview is getting a little complex in how it finds a piece of content's html:

sec/app/helpers/content_helper.rb

Lines 7 to 13 in 242e1ec

	if object.respond_to?(:summary?) && object.summary?
	html = object.summary
	elsif object.respond_to?(:body)
	html = object.body
	else
	html = object.description
	end

Currently articles, blog_posts, lessons, and managed_content use "body" as their main text field, while materials, topics, and uploads use "description". In addition, articles has a "summary" field that the rest lack. It would be nice if we migrated these tables so they all use the same name for this kind of column.