Trello-601: Dedicated Markdown Editing View on Card Back

The trello I grew to love used markdown for card text. Atlassian has decided to switch this on desktop for their own WYSIWYG editor with all the usual problems, especially when pasting formatted text.

I've heard claims this is because they're using a new backend format internally - on which I call BS because markdown is still the default on mobile and so both to and from markdown must be in the codebase.

(Markdown editing on desktop still works just fine with browser extensions, again giving the lie to claims it no longer works because of backend stuff.