Bug 16567: Correct RSS feed validity errors

Bug 16567: Correct RSS feed validity errors

This patch corrects a few markup errors in the XML feeds to make the
feeds validate correctly. The template has been largely re-indented to
improve readability and eliminate tab characters.

To test, apply the patch and perform a search in the OPAC.

- Open the RSS link appearing after the "Your search returned..."
  header.
- Depending on how your browser handles XML documents you may need to
  view source on the page to see the actual XML markup.
- Copy the source and paste it into the W3C feed validator:
  https://validator.w3.org/feed/#validate_by_input
- The feed should be found to be valid.
  - In my tests you'll get a "Recommendation" about "Self reference
    doesn't match document location." I think this recommendation
    doesn't apply.

Perform the same test with the other two available formats: Atom and
OpenSearch:

- Change the end of the RSS feed url from '&format=rss' to
  '&format=atom' and validate the result.
   - The same "self reference" recommendation will come up, and again I
     think we can ignore it.
   - There is another recommendation about "Two entries with the same
     value for atom:updated." The updated time is set to the same time
     as the feed itself (the time when the feed was generated. Maybe
     this is incorrect. If so we need a follow-up that exposes a
     different value to the template. biblio.datecreated maybe?
- Change the end of the feed url from '&format=atom' to
  '&format=OpenSearchDescription' and validate that result.

Signed-off-by: Brendan Lawlor <blawlor@clamsnet.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit ee960bd45f7535aec058d3984891e21b255ace12)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>