* Integrate GA4 code directly with `book.js`
The main advantage of this is that it simplifies the setup since we
can avoid the monkey-patching we did before.
A secondary advantage is that it should make things a little faster
since we avoid a request to the server on every page load.
* Remove unreachable return
* Watch all of `third_party`
It just occurred to me that we want to refresh the page in `mdbook serve` when anything changes in `third_party`.
We often see timeouts when running the course. Most of the time, the
code compiles on the second try, which indicates that we just need to
wait a little longer.
Fixes#406.
