Slightly improve favicon.ico.

Trim the whitespace so that the tube looks bigger while maintaining the aspect ratio.
Use File.exist? instead of deprecated File.exists?
Merge pull request #2213 from XhmikosR/docs-favicon
Update history to reflect merge of #2213 [ci skip]
Merge pull request #2214 from stomar/file-exists
Update history to reflect merge of #2214 [ci skip]
Adds reference to jekyll-page-collections plugin
Merge pull request #2215 from jeffkole/page-collection-plugin
Update history to reflect merge of #2215 [ci skip]
Move vendor prefixed properties before the unprefixed ones.
Use the shorthand font property when possible.

Conflicts:
	site/_includes/css/style.css
Be explicit with the `background` property.
Remove units from zero.
Remove duplicate properties.
Add fallback colors.
Add missing vendor prefixed properties.
Simplify selectors.