Use :root prefix to ensure correct background-image paths (#506)

Using embroider does not appear to use the same CSS import order as a regular Ember CLI build, and we haven't found a way to enforce it. While using `:root` to increase the specificity is certainly not great, it is probably better than using `!important` and appears to be the best workaround for now until we can find a better solution.

vendor/ember-phone-input.css

@@ -1,4 +1,5 @@
-.iti__flag {
+/* Using `:root` prefix here to increase the specificity to avoid a CSS import order bug when using embroider */
+:root .iti__flag {
   background-image: url('ember-phone-input/images/flags.png');
 }
 
@@ -8,7 +9,7 @@
   only screen and (min-device-pixel-ratio: 2),
   only screen and (min-resolution: 192dpi),
   only screen and (min-resolution: 2dppx) {
-  .iti__flag {
+  :root .iti__flag {
     background-image: url('ember-phone-input/images/[email protected]');
   }
 }