Why are these changes being introduced:

* We want to display a message in our restricted CDN auth flow that
  instructs a user to close the window/tab when the download is complete
  to return to their search if they came from TIMDEX UI.

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/GDT-317
* https://mitlibraries.atlassian.net/browse/GDT-304

How does this address that need:

* Adds `target=_blank` to download link tags
* appends `?timdex-ui=true` to download link URLs

Document any side effects to this change:

* A bunch of tests reformatted automatically on save. The only relevant
  changes to the tests were on lines 246-247

A few complex lines were moved to their own methods to make reading the method a bit easier