Draft: Implement rich text authoring using SimpleMDE

This is a Proof of concept PR to add support for authoring replies in rich text. It is able to provide preview of the rendered email on a sidebar, as a live preview.

This relies on server side rendering and hence requires to make API call on each time renderer is invoked. However, the editor itself does come with a renderer and there are some styles applied directly to the editor too, which can be slightly distracting since the rendering rules can be different for server.