Don't require the `scope` key in front matter defaults

Set defaults for `scope` in front matter defaults as per #2656
Use .scss for site_template styles.
Fix broken links now that docs is a collection in site.
Be more ambiguous about 'page.content'. But more transparent.

Fixes #2522.
Update history to reflect closing of #2522.
Only include lib files in the gem

Resolves https://github.com/jekyll/jekyll/issues/2670
Update to a better, more gitty way
Merge pull request #2671 from gjtorikian/patch-1
Update history to reflect merge of #2671 [ci skip]
Don't include test_files in gemspec. #2671.
Replacing front-matter/frontmatter with front matter
Fixing anchor references to #front-matter-defaults
Fix `git diff` command in CI script
Merge pull request #2672 from alfredxing/fix-ci-git-diff
Update history to reflect merge of #2672 [ci skip]
Merge pull request #2667 from jekyll/use-page
Update history to reflect merge of #2667 [ci skip]
Add tests for front matter defaults
Merge pull request #2659 from alfredxing/front-matter-defaults-defaults
Update history to reflect merge of #2659 [ci skip]