DS-1483 Generate citation_pdf_url in more cases

[aschweer]

Google Scholar uses the citation_pdf_url header tag on item pages to determine
which of an item's bitstreams to link to (if any). Standard DSpace behaviour is
to include this link only if the item has exactly one bitstream in the ORIGINAL
bundle and if this item is of type application/pdf. The desired behaviour for
the LCoNZ IRRs is this:

• if the item has only one bitstream in the ORIGINAL bundle, link to this one
• if the item has a primary bitstream in the ORIGINAL bundle, link to this one
  (regardless of how many bitstreams there are in total in the ORIGINAL bundle).
  Additionally, the bitstream link should only appear when the bitstream selected
  according to the rules above is publicly visible.
  The mimetype of the bitstream is not taken into account, as per Google Scholar
  feedback that this is no longer as important to them.

[tdonohue]

One quick note, @aschweer

Can you add the associated JIRA ticket id (DS-1483) to this PR? I'm unsure what the "#1887" refers to (as this ticket number doesn't exist in DuraSpace JIRA).

Beyond that, I'd love to see this get into 4.0. Need to test it myself still.

[aschweer]

Oops good point @tdonohue -- it still had the issue number from our internal tracker on it. Fixed.

[aschweer]

Just a note that I ended up force-pushing so that the Jira ticket id is on the actual commit as well. I hope this doesn't cause problems for anyone.