Added styles for new calc and updated bourbon

static/css/application.css

static/css/marketing.css

static/images/calc-icon.png

static/images/close-calc-icon.png

templates/main.html

templates/sass/application.scss

templates/sass/base/_functions.scss

templates/sass/bourbon/_bourbon.scss

templates/sass/bourbon/addons/_clearfix.scss

templates/sass/bourbon/css3/_background-image.scss

templates/sass/bourbon/css3/_border-image.scss

templates/sass/bourbon/css3/_border-radius.scss

templates/sass/bourbon/css3/_box-shadow.scss

templates/sass/bourbon/css3/_box-sizing.scss

templates/sass/bourbon/css3/_linear-gradient.scss

templates/sass/bourbon/css3/_radial-gradient.scss

templates/sass/bourbon/css3/_user-select.scss

templates/sass/bourbon/functions/_flex-grid.scss

templates/sass/bourbon/functions/_render-gradients.scss

templates/sass/discussion/_modals.scss