Download an html grade of the user's tutorial.
Shiny ui and server logic for the grade computation.
Note that when including these functions in a learnr Rmd document it is necessary that
the server function, grade_server(), be included in an R chunk where context="server" as
they interact with the underlying Shiny functionality. Conversely, the ui function,
grade_print_ui(), must not be included in an R chunk with a context.