MVC: Markdown preview side-by-side in the Web IDE

Problem

Currently a tab click is needed to see your rendered markdown.

Proposal

Pull out the preview tab to be a side by side view when a .md file is detected. It should have the following attributes:

• Only show up when a .md file is selected in the Web IDE
• Defaulted on (shows up)
• Can be collapsed to the right hand side bar, similar to the terminal
• Can be dragged to (similar to the diff view in the Web IDE) to devote more or less screen space to each panel
• Button should exist at the top preview pane which will refresh the rendered markdown
• Only works for .md files

Links

• https://josephernest.github.io/writing/
• https://ghost.org/features/

cc: @timzallmann @iamphill @jramsay