riddlr quizes

Add in a "quiz" as a series of riddles with a global clock that persists through all riddles.

What this needs
- A new class representing the collection of riddles and additional quiz parameters
- whether a user should be able to do mulitple submissions
- whether a user should get feedback about whether their solution is correct
- global clock
- A way of producing a quiz shiny module
- which adds a submission callback that automatically redirects to the next question
- possibly with quiz question table-of-contents; would be awesome if the table of contents was reactive to quickly display whether the question is skipped, wrong or correct.
- somewhere where the time remaining is displayed; display could turn red when X% of time is remaining
- A yaml template to make it easy to specify a quiz from a collection of riddles