Make shiny modules for quiz questions (instead of htmlwidgets)

[schloerke]

Fixes #192
Fixes #173
Fixes #109
Fixes #217

Will make a separate PR for text box input-based questions (included)

[jcheng5]

1. Looks like there are some TODO-barret comments left, do any of those need to be addressed?
2. Are you aware of any backward-incompatible changes in this PR? I've only started skimming but I vaguely recall you saying you were getting rid of the quiz function?

[jcheng5]

To (partially) answer my own question, the question() arguments correct and incorrect have been renamed to correct_message and incorrect_message. Do you really want to break backward compat for that? (The examples still use the old param names btw)

[jcheng5]

I can't get this to work in RStudio. It seems to me like the S3 methods are not registered properly. This is what I'm testing with:

---
title: "learnr test"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)
```

```{r one}
question("Where are you right now? (select ALL that apply)",
 answer("Planet Earth", correct = TRUE),
 answer("Pluto"),
 answer("At a computing device", correct = TRUE),
 answer("In the Milky Way", correct = TRUE),
 incorrect_message = paste0("Incorrect. You're on Earth, ",
                    "in the Milky Way, at a computer.")
)
```

Instead of the report launching, I get each individual answer launching in a new window. If I use devtools::load_all I get different behavior, where the structure of the question is dumped into the output as text.

Update: Oh, interesting. I had some local changes to htmltools that caused this. Seems to almost be working now, other than the missing import declarations I mentioned below.

[jcheng5]

Also there seem to be a lot of missing import declarations, R CMD check will point them out.

[jcheng5]

I'll try again after you get the right imports in place so it works in RStudio.

I'll have some feedback about question_module_server as well, we can discuss that after I am actually able to run tutorials.