Replace deprecated Ruby methods

Replace `Hash#has_key?` and `File.exists?` with `Hash#key?`
and `File.exist?`
improving strftime call

set non padded config to avoid parse string type twice
change links from Markdown to HTML
fix quote
Merge pull request #2679 from budparr/master
Update history to reflect merge of #2679 [ci skip]
Merge pull request #2673 from lucianosousa/patch-1
Update history to reflect merge of #2673 [ci skip]
Merge pull request #2664 from alfredxing/replace-deprecated-methods
Update history to reflect merge of #2664 [ci skip]
Add helpful comments to site template _config.yml
Add friendly error message for invalid dates

Catch the `ArgumentError` when parsing dates from the YAML front
matter
Use `relative_path` in error message instead of `path`
No longer set "permalink: pretty" in the _config.yml for the site template

This causes unintuitive behaviour that doesn’t align with the
documentation.

Fixes #2399.
Merge pull request #2680 from jekyll/no-pretty
Update history to reflect merge of #2680 [ci skip]
Merge pull request #2649 from alfredxing/friendly-date-error
Update history to reflect merge of #2649 [ci skip]