Provide ability to edit documents on the Hub

[alex-ketch]

In order to reduce friction in user workflows, and avoid them having to download>edit>upload documents, we would like to allow them to directly edit and save changes to their project files in the browser.

This issue does not include:

• sync changes back to the source locations (e.g. GitHub, Google Drive, etc.)

Tasks:

• Review and refine UX/UI if needed
• Evaluate text editors and integrations costs involved (VS Code vs Designa Editor/Code Mirror)

[alex-ketch]

A user suggested having the ability to create a new plain text file when adding a new project source. The desire there being to immediately start writing or testing snippets, copy-pasting things over, etc.
I think it would make sense to implement this functionality as part of this issue.

[alex-ketch]

Designs can be viewed here: https://www.figma.com/file/xGmRJ6kGlZ4VkI4fJtEO4a/File-Editing?node-id=0%3A1