Use predictable column names for content

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

https://github.com/EFForg/sec/blob/242e1ec95608130f08d13d493da8b6f6434b6e0f/app/helpers/content_helper.rb#L7-L13

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.