Extract times from slides and generate a schedule

[djmitche]

Once the v2 work lands (#1073), each slide will have some "markdown frontmatter" containing a minutes: .. property suggesting the time that the slide should take.

---
minutes: 10
---

It would be great to be able to summarize the total time required per-segment (segments are the level of division within each morning/afternoon), and then including that in a single "schedule" slide at the beginning of the course. That schedule would be useful for setting student expectations, and also something we can keep an eye on for purposes of preventing the course from getting too long!

One choice to make here is whether to generate output that gets checked in, or to write an mdbook preprocessor that generates the output as mdbook is running. An advantage of checking in the output would be seeing schedule changes in GitHub pull requests. But a disadvantage is that it's one more thing that contributors need to remember to do before making a PR.