closes#1807
- e2e tests now also cover the tax calculator.
- Dart app updated to match TS (it had no sales tax calculator).
- TS sample source cleanup (e.g. removed many unnecessary `docregions`).
- Prose updated to include @kwalrath's revisions from a while ago, Ward's comments, and
some of my edits as well.
Contributes to #1598 and #1508.
