Convert markup of the posts on upgrade

@lukaseder has already added the function to convert wiki markup into GitHub syntax, but more can be done for it.

Reference: [Trac's WikiFormatting](http://trac.edgewall.org/wiki/WikiFormatting) vs. [Markdown (used by GitHub)](http://daringfireball.net/projects/markdown/syntax/).

I don't think all of those entities have to be converted, but rather the basic ones like formatting and links.
