Replace Pandoc AST format with HAST/mdast

[tumidi]

Idea: Replace Pandoc AST with something like HAST or mdast.

We would need to migrate to Generic directives which seems more standard-like, features inline, leaf and block directives which innoDoc Markdown syntax would benefit from.

Problems:

• innoconv would need to be implemented in JS.
• webapp would need to be changed quite a lot.
• Conversion from LaTeX would be more complicated as we would somehow convert from LaTex->Pandoc->Pandoc AST->?->Markdown. Currently we rely on the fenced_div extension to implement special content like cards, questions etc. (Pandoc PR: Adjust fenced_divs syntax to resemble Markdown directive syntax)

One idea is to completely remove innoconv and parse Markdown sources on-the-fly in the webapp server.