Use SSL for links and assets where possible

I was interested in what it would take to move the site to HTTPS, so I ran the app using [ngrok](https://ngrok.com/) and fixed up some mixed content warnings by turning various `http://` URLs into `https://` URLs. I also did a scan of lots of other `http://` URLs used in different data objects and stuff, and turned them into `https://` where appropriate.

I only touched index, layout, and data stuff, not individual post pages, so there may be mixed content warnings from images/embeds in individual posts.

The main thing I couldn't fix was the linkrot API call, which is not available over HTTPS (and which seems to 500 out when not POSTed to from a whitelisted domain).

![errors](https://cloud.githubusercontent.com/assets/4592/4345551/a517e7ec-40d4-11e4-9730-fc6af7cc2f21.png)

(The 404 is a separate problem, and exists on the live site already.)