This amends it so sanitize always uses the most strict sanitizer
by default, if it needs to be "widened" to use more options then
you need to pass it more options.
This adds the markdown.it engine to Discourse.
https://github.com/markdown-it/markdown-it
As the migration is going to take a while the new engine is default
disabled. To enable it you must change the hidden site setting:
enable_experimental_markdown_it.
This commit is a squash of many other commits, it also includes some
improvements to autospec (ability to run plugins), and a dev dependency
on the og gem for html normalization.
