Fix validation problems

[pkra]

We're invalid, e.g., http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fmathblogging.org%2Fposts.xml

• prevent duplicate items
  • example: a Terry Tao post appears twice. I have no idea why and would like to understand
  • We should filter out duplicates by guids during feedmerging.
• prevent invalid feeds from invalidating our content
  • See NotAbstractNonsense; I've reached out via twitter to get them to fix it
  • we need to fix invalid items. Might be worth reporting/discussing upstream with feedparser dev.
• Let's do a pre-mortem: next time the feed is invalid, what might have gone wrong?

[pkra]

Closed for https://github.com/mathblogging/mathblogging-base/issues/40