Don't try to render unavailable tweets in RSS feeds

Fixes #868.

Conflicts:
  src/views/rss.nimf

src/views/rss.nimf

@@ -84,6 +84,8 @@ Twitter feed for: ${desc}. Generated by ${cfg.hostname}
 #
 #    let retweet = if tweet.retweet.isSome: tweet.user.username else: ""
 #    let tweet = if retweet.len > 0: tweet.retweet.get else: tweet
+#    if not tweet.available: continue
+#    end if
 #    let link = getLink(tweet)
 #    if link in links: continue
 #    end if