FontAwesome
参考：
Icons | Font Awesome [Official]
Cheatsheet | Font Awesome [Official]
FortAwesome/Font-Awesome: The iconic SVG, font, and CSS toolkit – GitHub
@fortawesome/fontawesome-free – npm
ドキュメント
参考：
Using a Package Manager | Font Awesome [Official]
FontAwesome 4.7
参考：
Font Awesome Icons v.4.7 [Official]
Font Awesome Examples v4.7 [Official]
FortAwesome/Font-Awesome at fa-4 – GitHub
Font Awesome 4.x のアイコンの class 名を確認するときのページ | FlipClap
font-awesome-sass
参考：
FortAwesome/font-awesome-sass: Font-Awesome Sass gem for use in Ruby/Rails projects – GitHub
Incompatible with sassc-rails · Issue #112 · FortAwesome/font-awesome-sass – GitHub
Use sassc for 4.7.0 by p8 · Pull Request #172 · FortAwesome/font-awesome-sass – GitHub
font-awesome-rails
参考：
使い方
参考：
CSS Pseudo-Elements | Font Awesome [Official]