Improve CDE display in Dug

[gaurav]

@YaphetKG recommended two changes:

1. Instead of having file downloads be separate nodes, add them to the CRF publication directly as attributes. There should be general attributes by file type (e.g. msword, msexcel, pdf, etc.) and then these should be subsetted by language (e.g. msword-en, msword-es, etc.). There should be a catch-all files (and files-en, files-es, etc.). Attributes can be arrays, so that can be used to write a list of attributes if that is needed.
2. Add a CDE count so we know how many CDEs are present in each CRF. We could add a cde-count attribute, but it'd be quicker and hackier to add it to the end of the summary (e.g. Contains 12 CDEs.)

He also suggested proposing biolink:Measure (or questionnaire, instrument, CRF, etc.) as a list of biolink:CommonDataElements.