Add new alert icons (#2226)

src/pretix/static/pretixbase/scss/_variables.scss

@@ -6,6 +6,10 @@
   @return mix(black, $color, $percentage);
 }
 
+@function url-friendly-colour($colour) {
+    @return '%23' + str-slice('#{$colour}', 2, -1)
+}
+
 $gray-darker: lighten(#000, 13.5%);
 $gray-dark: lighten(#000, 20%);
 $gray: lighten(#000, 33.5%);