Add svg icon mixin

client/scss/tools/_mixins.general.scss

@@ -106,6 +106,11 @@
     margin-right: 0.2em;
 }
 
+@mixin svg-icon ($size: 1.5em) {
+    width: $size;
+    height: $size;
+    vertical-align: text-top;
+}
 
 // Applies given rules on hover, except for touch screens.
 // Relies on feature detection to add a no-touch class on the html element.