Switch to a different markdown parser

[rubenwardy]

Problem

ContentDB currently uses python markdown, which aims to implement markdown exactly as written in the original document. This causes a lot of confusion with users, as most sites use a less strict version of Markdown such as GitHub Flavored Markdown or Common Markdown. For example, one error that catches users out is that blank lines are required around blocks (paragraphs, lists, code blocks, etc)

Solutions

Switch to a less strict markdown parser, such as GitHub Flavored Markdown or Common Mark

Will need to test with existing packages to make sure that breakages aren't too severe