Fix html_filter and markdown_filter using absolute paths to images

This made hashes in tests dependent on sorl-thumbnail's root. See comments on https://github.com/mariocesar/sorl-thumbnail/pull/330 . Also fix corresponding hashes. There is a bigger issue here: https://github.com/mariocesar/sorl-thumbnail/issues/283 , but I would propose to do the opposite: whenever there is a possibility, make paths relative to MEDIA_ROOT. This allows copying everything in MEDIA_ROOT to another machine with a different MEDIA_ROOT for example, and avoids issues like this with the tests.