Multiple CMS improvements:

* Use unichr() instead of chr() to be Unicode-safe
* Allow Markdown in HTML blocks
* Allow defining per-repository template filters
* Don't convert page-relative links
* Don't throw an exception for missing directories (test server only)
* Case-insensitive comparison of file extensions
* Serve correct MIME type for JPEG images in test server

Review: http://codereview.adblockplus.org/5567002995326976/