Many files meant to be used as includes had somehow been transformed
into standalone HTML files.

The extra header and footer boilerplate has been removed, and
single-quoted attributes have been turned into proper double-quoted
attributes.

W. Trevor King: I dropped everything from the original 6a4f3fb except
for the LICENSE.md and lessons/swc-python/ears.py additions.  I also
added a blank line to Greg's commit message for cleaner 'git log
--oneline' formatting.

Conflicts:
_includes/banner.html
_includes/bootcamps/contact.html
_includes/bootcamps/eventbrite.html
_includes/bootcamps/instructors.html
_includes/bootcamps/python.html
_includes/bootcamps/r.html
_includes/bootcamps/requirements.html
_includes/bootcamps/what.html
_includes/bootcamps/who.html
_includes/footer.html
_includes/guide-db/aggregate/challenges.html
_includes/guide-db/aggregate/challenges/average-null.html
_includes/guide-db/aggregate/challenges/concat.html
_includes/guide-db/aggregate/challenges/count-ave-temp.html
_includes/guide-db/aggregate/challenges/diff-with-average.html
_includes/guide-db/aggregate/keypoints.html
_includes/guide-db/aggregate/lesson.html
_includes/guide-db/calc/challenges.html
_includes/guide-db/calc/challenges/adjust-salinity.html
_includes/guide-db/calc/challenges/missing-units.html
_includes/guide-db/calc/challenges/union.html
_includes/guide-db/calc/challenges/unique-major-site-ids.html
_includes/guide-db/calc/keypoints.html
_includes/guide-db/calc/lesson.html
_includes/guide-db/create/challenges.html
_includes/guide-db/create/challenges/dump-version-control.html
_includes/guide-db/create/challenges/insert-olmstead.html
_includes/guide-db/create/challenges/replace-null.html
_includes/guide-db/create/keypoints.html
_includes/guide-db/create/lesson.html
_includes/guide-db/distinct/challenges.html
_includes/guide-db/distinct/challenges/distinct-dates.html
_includes/guide-db/distinct/challenges/distinct-vs-sort-uniq.html
_includes/guide-db/distinct/keypoints.html
_includes/guide-db/distinct/lesson.html
_includes/guide-db/filter/challenges.html
_includes/guide-db/filter/challenges/fix-selection.html
_includes/guide-db/filter/challenges/out-of-range.html
_includes/guide-db/filter/challenges/select-like.html
_includes/guide-db/filter/keypoints.html
_includes/guide-db/filter/lesson.html
_includes/guide-db/grouping/challenges/date-range.html
_includes/guide-db/grouping/challenges/simplify-query.html
_includes/guide-db/grouping/challenges/trace-pipeline.html