Last lesson in series links to non-existent following lesson

[terichadbourne]

After hitting "Submit" and successfully completing lesson 3 of the "Basics" series (https://proto.school/#/basics/03) , the "Submit" button turns into a "Next" button which, on click, tries to advances to a non-existent lesson 4 in this series (https://proto.school/#/basics/04), which loads as a blank white page.

We need a way to mark the last lesson in a series so that it will prevent the default behavior and instead either (a) advance to the next series, if it exists, or (b) go back to the main workshop page so the user can select another lesson series to try.

[terichadbourne]

duplicate of #38

[terichadbourne]

Closed by merging of PR #63