Require connected GitHub account

[egillespie]

As a user, I want to connect my GitHub account the next time I sign in to Code Lab so my regular coursework is not interrupted mid-workflow.

Acceptance Criteria

• Any user without a connected GitHub account is redirected to the "Get Started" page
• Any user with outdated GitHub permissions is redirected to the "Get Started" page
• The "Get Started" page shows a "Connect to GitHub" section explaining why a user without a connected GitHub account must connect to GitHub before using the app
• The "Get Started" page shows a "Reconnect to GitHub" section explaining why a user with outdated GitHub permissions must reconnect their GitHub account before using the app
• The "Connect to GitHub" and "Reconnect to GitHub" sections on the "Get Started" page provide a button that allows a a user to (re)connect their GitHub account
• The "Get Started" page prevents a user from leaving the page until all required items ("Enable Notifications", "Connect to GitHub", "Reconnect to GitHub") have been completed