Sanitize using Forem::Sanitizer

Using `ERB::Util.h()` was good for a time, until it started escaping *everything and anything*...

So in Forem we now have a `Forem::Sanitizer` class which delegates out to the sanitize gem to sanitize output. Using the sanitizer within this formatter fixes [point 3 in this comment](https://github.com/radar/forem/issues/359#issuecomment-28790213).

Thanks!