Download a FileSet's ExtractedText file

[conorom]

• Enable the downloading of a FileSet's ExtractedText file
• use extracted_text as the file URL param

This ties in with #1261

[mbakeryo]

I've put together a pretty lame example of multiple format types on one asset page with the OCR disclaimer. More to do here, but this is a start to a document to try and determine the best way to move forward: https://docs.google.com/document/d/1KDBh5jCth5R53MUPli8ZjQxy1asfN9KE13-EaM7X_hg/edit.

I appreciate any comments you may have.

This is also related to #1429

[mbakeryo]

This is blocked by Apex - waiting for the final files.

[conorom]

I'm reissuing a rebased PR for this work where the method that decides whether the FileSet's extracted_text association has a non-zero-size file available always returns false in production. I've tested that the download button no longer appears on staging assets with extracted text.

If nothing else, the code provides a place to work from if and when we want to add the downloading of custom/new FileSet associations (accessible_version or what have you).