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

Set defaults for `scope` in front matter defaults as per #2656
Replace deprecated Ruby methods

Replace `Hash#has_key?` and `File.exists?` with `Hash#key?`
and `File.exist?`
add warning flag to note
Use .scss for site_template styles.
fix warning flag (move from wrong note)
add back info class
improving strftime call

set non padded config to avoid parse string type twice
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]
Merge pull request #2674 from kleinfreund/master
Update history to reflect merge of #2674 [ci skip]
Explain where Main files go and whereone can find an example site.