Fix #376: Intermittent crashes in RSSContent.cache_content

Sometimes, the feeds seem to have no `link` element. Stop the
update_rsscontent management command from crashing hard when this
happens.

Thanks to Piet Delport for the report!
Blog entries do not have a redirect_to attribute
Merge branch 'master' into next

* master:
  Blog entries do not have a redirect_to attribute
  Fix #376: Intermittent crashes in RSSContent.cache_content