RSS: validation issues

[infinite-persistence]

• Most validators now perform an extra encoded-url check. We are serving 404 html for those cases
  • 
• itunes issues https://help.apple.com/itc/podcasts_connect/#/itcb54353390
  • itunes:explicit must be "true" or "false" now, but we are using "no"
  • language now in the required section

[sktbrd]

I've submitted a PR for this issue: #3432

The fix changes itunes:explicit format from 'yes'/'no' to 'true'/'false' to comply with current iTunes/Apple Podcasts Connect requirements.