Changed content controller's default response type to HTML

The Facebook crawler sends requests with the header `Accept: */*`, which currently makes the content controller respond with JSON. See [this stack overflow question](http://stackoverflow.com/questions/12094683/facebook-open-graph-action-rails-return-json-or-html).

This PR changes the default response content type to `text/html`, which seems reasonable for a CMS. Hopefully anyone making requests for JSON is specifying `application/json` in their `Accept` header.

EDIT: you can see what Facebook sees using their [URL linter](https://developers.facebook.com/tools/debug/).