Main text: implement most of David's changes

This commit implements most of David's changes from c76727b, but excluding
some, such as the proposal to use 'which' in a restrictive clause in the
abstract. This is allowed, but the Fowler brothers' rule tends to followed
in science writing:
https://ell.stackexchange.com/questions/5/is-there-any-difference-between-which-and-that

A few points on the abstract:
  * an immediate solution = singular
  * The "immediate, fast short-term" benefits sentence sounded like it was
    redundantly superfluously repetitively repeating doubled-up
    information. Hopefully this edit is better.
  * in the %Conclusion, "solutions" is vague, like people who say
    "technology" when they're only talking about software, so this edit
    reminds the reader to make the sentence more self-contained and
    understandable.

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