Fix clojure formatting in comments

There is a problem with formatting of clojure code not only in submissions (like already discussed #821) but in all comments as well. Solution is the same: use `Loofah::fragment` instead of `Loofah::xml_fragment`.

To achieve this I made changes to `ConvertsMarkdownToHTML` and reused it in helper which generates highlighted HTML for submission.

But since all comments are converted to HTML on creation it is necessary to regenerate fixed HTML. What is the best way to do it? Rake task which iterates over comments in Mongo and fixes them?