[ENG-9046][followup] fix rss/atom validation errors

[aaxelb]

fix validation errors found on index-card-search rss/atom feeds

• atom renderer (see validation report, now fixed)
  • <link href="..." /> instead of <link>...</link>
  • add <updated> feed element
  • fix missing <author>
  • only one <uri> per <author>
  • include filename as additional title
• rss renderer (see validation report, now fixed)
  • use dc:creator instead of author (which can occur only once and requires email address)
  • use filename for title when title missing
  • add self-link <atom:link rel="self" ...
• also add feed links to plain json rendering

[coveralls]

coverage: 84.4% (+0.1%) from 84.29%
when pulling 8d48649 on aaxelb:feat/9046/shtrove-rss-followup
into 71ebed7 on CenterForOpenScience:develop.

[felliott]

Seems reasonable to me. Approved!

Cheers,
@felliott