Feed item saving should fall back to webpage saving

[dstillman]

I don't remember if we're currently supposed to be falling back to webpage saving when there's no translator for a feed item's URL, but we should.

Currently on http://www.inderscience.com/offer.php?id=77320, from the feed http://www.inderscience.com/current_issues/ijhvs.rss, a saving box shows up and then stays open, with an error in the console about there not being any translators.

Adding this just to track it, but instead of adding dedicated functionality here, this seems like another argument in favor of creating a generic webpage translator that works on any webpage, which I suggested in zotero/translation-server#32.

Reported here: https://forums.zotero.org/discussion/60741/