Add Link Tracks to Download Hrefs

[merwhite11]

Description, Patron Story

Add data-ol-link-track="Download" to Download options on books page. Observing click rates in the download option's current state will give a baseline comparison which we can use to the test the efficacy of an additional 'Download' option placement in the Read/Borrow button (@SivanC working on in #5831)

Proposal

Add link tracks to all hrefs in the following files:

• [x] ia_download_options
• [x] cita_press_download_options
• [x] gutenberg_download_options
• [x] librivox_download_options
• [x] openstax_download_options
• [x] standard_ebooks_download_options
• [x] wikisource_download_options

Feedback Questions

Decisions

Stakeholders

@cdrini @SivanC