Many files meant to be used as includes had somehow been transformed

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