Use new content model to write plain language metadata descriptions

[jenniferthibault]

The designs for the new Advanced data section integrate the downloadable data formerly found in the data catalog and FPT files. For the first release, all descriptive content was pulled from the classic site. This means it was written by data experts. In order to make it more approachable to a broader range of users, we'd like to rewrite this content using plain language according to the content model designed in https://github.com/18F/fec-cms/issues/1402

Content model

• (to come)

Completion criteria

• (to come)