Link lesson notes to tutorial/lesson URL

[jrobind]

After discussion in the Discord channel, to make lessons easier to reference and discover we should (optionally) allow users to link lessons to a relevant tutorial URL.

Aside from the obvious and merely adding the tutorial URL to the lesson notes themselves, It would make more sense to add this option as a new input field.

This ticket should cover:

• add optional URL input field
• make sure this new data is correctly stored and available to all lessons