Bug 37679: Add DC subtypes to allow-list in opac-export.pl

Bug 37679: Add DC subtypes to allow-list in opac-export.pl

Bug 37370 added an allow-list of formats for exporting in the OPAC. But
it missed the fact there was a modal for choosing DublinCore subtypes
when Dublin Core is chosen in the first place.

This patch fixes this.

To test:
1. On a fresh KTD, make sure `OpacExportOptions` has all options enabled
2. Go to the OPAC and pick a record detail page
3. Choose Save record > Bibtex
=> SUCCESS: The save dialog for bibtext works
4. Repeat for DublinCore, choose any of the sub formats in the modal
=> FAIL: You get a 400 page
5. Go back to 2
6. Apply this patch, and repeat 4
=> SUCCESS: It works!
7. Without refreshing the page, disable DublinCore in the sysprefs
8. Choose any subformat and proceed
=> SUCCESS: You get a 400! This means the subformats are only considered
if DublinCore  itself is enabled
9. Sign off :-D

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>