feat: Add button to move to next lesson at end of each one

[kempsterrrr]

When scoping out #235, I realised that when a user completes a lesson, they are taken back to the lesson page, and there are no instructions for what the user should do next. This experience can be improved to provide the user more guidance on completing the track they're taking or what to do if they've completed the track.

In most cases, this will be "move on to the next lesson"; however, at the end of a track, let's send users back to the tracks page so they can find another track to take.

I suggest we implement this feature in two ways:

1. Once Feature: Social share when completing lesson quiz #235 is complete and the modal shows after lessons are completed, replace the "DONE" button with a button that takes the user to the next lesson
2. Show a button at the bottom of the lesson that allows the user to navigate to the next (and maybe previous) lesson

Example in success Modal

Example at end of lesson

• As a user, I want to know what to do next when I have completed a lesson
  • Update the "DONE" button to show the relevant next step, i.e. "Go to next lesson."
  • Add "Previous Lesson" and "Next Lesson" at the button on the lesson page
  • For the last lesson in a track, the "Next Lesson" button should show "Try another Track" (or similar) and take the user back to the /tracks page

[kempsterrrr]

I've partially delivered this feature by adding a Next Lesson button in #240. I did not add a Previous Lesson button as there was not enough space on the modal to do this. It might make sense to think about this feature more before adding anything extra cc @uma-l