Add basic support for client extensions. Closes #780

[brollb]

This is a very minimal framework for client extensions that is using an autograder as a motivating example. There are a few remaining questions/tasks before this could be merged:

• Should we expose a custom endpoint with "official" extensions and make them able to be loaded from within the environment? Currently the autograder is hard coded but this is certainly not ideal.
• load extensions via URL
• add custom blocks to the extension API?

[brollb]

A few more thoughts (some of these would probably be best to have in their own PR):

• Configure extensions via URL (custom autograders)
• autograde tasks like "draw a square/house" or "ticking clock"
  • how to track state in a way that is invariant to some aspects but not others...
• Custom/additional stages
• additional tabs