Add ability to preview documents as styled HTML without having to create a Snapshot

[alex-ketch]

The current user workflow for authoring a document can be cumbersome when attempting to refine the syntax and stylistic issues. They currently have to download a file, edit locally, upload, start generating a Snapshot, and wait for it to complete before they can view it.
We'd like to reduce the time it takes between making a change, and seeing the effect it has.

To do so, we'd like to allow previewing documents as themed HTML in the browser without having to generate a Snapshot first.

Outstanding tasks:

• Design UI/UX and how it fits into proposed changes in Provide ability to edit documents on the Hub #959
• Make it happen :)

Related to: #659, #786, #959