Instructor-led workshop UI

[RedBrogdon]

Design and build a new playground-style page to be used for step-by-step codelab workshops.

Basic features:

• Create a new, full-screen page that can accept a codelab's name, location, or other identifying info in its query string.
• On first load, automatically place the initial code state for the workshop into the editor.
• Provide a UI control for advancing and retreating through the steps of the codelab.
  • Each step will have an associated code state, which should automatically be placed into the editor.
• Offer the ability to load the codelab's code from a non-rate-limited source.
  • This means somewhere other than GitHub, essentially. We may need to implement "load from arbitrary URL" to make this happen.

To do:

• Initial UI setup (Codelab UI - initial setup #1807)
• Authoring guide
• Verify Cloud Storage hosting
• GitHub integration (Add Github codelab fetcher #1811)
• "Show solution" button (Add show solution button to Codelab UI #1819)
• Colorized code snippets in instructions panel (Add highlighting to snippets diplayed in codelab instructions panel #1829)
• Analyzer / completions (Add more workshop features #1834)
• Documentation panel (Add more workshop features #1834)
• Hide UI Output in Dart mode (Add more workshop features #1834)
• keyboard shortcuts (Add more workshop features #1834)
• Share more code with playground UI (Upgrade to Dart 2.0.0 #845)
• Format button (Add more workshop features #1834)
• Run snippet when the step changes
• Get hosting URL for codelabs
• Add CI to DartPad codelabs in flutter/codelabs
• Loading UI

Might do:

• three column layout
• query parameters for each step (?step=2)