Use Rails asset pipeline URL functions to load icon font if they exist (#737)

Author: mgreen-lz

src/sass/icons/_mixins.scss

@@ -2,13 +2,23 @@
 
 $context-menu-icons: () !default;
 
+@function context-menu-font-url($url) {
+  @if function-exists(asset-url) {
+    @return asset-url($url);
+  } @else if function-exists(font-url) {
+    @return font-url($url);
+  } @else {
+    @return url($url);
+  }
+}
+
 @font-face {
   font-family: '#{$context-menu-icon-font-name}';
-  src: url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.eot?#{$context-menu-icons-cachebust}');
-  src: url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.eot?#{$context-menu-icons-cachebust}#iefix') format('embedded-opentype'),
-  url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.woff2?#{$context-menu-icons-cachebust}') format('woff2'),
-  url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.woff?#{$context-menu-icons-cachebust}') format('woff'),
-  url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.ttf?#{$context-menu-icons-cachebust}') format('truetype');
+  src: context-menu-font-url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.eot?#{$context-menu-icons-cachebust}');
+  src: context-menu-font-url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.eot?#{$context-menu-icons-cachebust}#iefix') format('embedded-opentype'),
+  context-menu-font-url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.woff2?#{$context-menu-icons-cachebust}') format('woff2'),
+  context-menu-font-url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.woff?#{$context-menu-icons-cachebust}') format('woff'),
+  context-menu-font-url('#{$context-menu-icon-font-path}#{$context-menu-icon-font-name}.ttf?#{$context-menu-icons-cachebust}') format('truetype');
   font-weight: normal;
   font-style: normal;
 }