Extension system design

[luxaritas]

Marking this as blocked as this will likely depend heavily on other systems, though not sure when we want to start digging into this.

As a note for myself, a couple things we need to account for:

• Utility functions
• Custom UI (dialogs, etc)
• State change
• API requests (and we'll need some sort of oauth-style flow to generate an access token with limited user-approved features)