Mentions inside Markdown break MD state

[KitsuneRal]

There's still a fundamental problem concerned with putting mentions inside Markdown, in that those mentions effectively split the Markdown document into two. This means that mentions inside lists or even between backticks break the state of MD parser, leading to various unpleasantries depending on the case: when in a list, the mention ends up being after the list instead; backticks are not paired together, meaning the piece of text is no more preformatted; and so on.

The proper fix for that would of course be to pass the whole thing to the parser; but its current quality doesn't allow that.