Add Sass mixins and use them.

Reduces code duplication and makes things cleaner.
Move custom code from _font-awesome.scss to _style.scss.

Also change the order of includes so that _style.scss is last for precedence reasons.
Adding Webmentions in the right place this time :-)
Merge pull request #3127 from aarongustafson/master
Update history to reflect merge of #3127 [ci skip]
Merge pull request #2904 from XhmikosR/mixins
Update history to reflect merge of #2904 [ci skip]
Update gridism.