add: separate markdown previewer for post and reply publishing forms

[plebeius-eth]

https://t.me/seeditreact/8076 reddit doesn't have this feature but I think it's very helpful. A "see preview" button can temporarily hide the textarea and replace it with the markdown rendering of the comment, to show how it will look like. Clicking the button again ("hide preview" or maybe "back") should show the textarea again. This can be added to both post and reply forms.