Move flash mixin into variables file

src/assets/scss/variables.scss

@@ -26,6 +26,27 @@ $color__green--darker: #43A047;
 $color__green--darkest: #1B5E20;
 $color__green--darkester: rgba(79, 152, 82, 0.84);
 
+@keyframes flash {
+	0% {
+		background-color: $color__gray--darker;
+	}
+	50% {
+		background-color: $color__lightgray--darkest;
+	}
+	75% {
+		background-color: $color__gray--primary;
+	}
+	to {
+		background-color: $color__gray--darker;
+	}
+}
+
+@mixin flash {
+	animation-name: flash;
+	animation-duration: 1s;
+	animation-iteration-count: infinite;
+	animation-timing-function: linear;
+}
 
 @mixin text($family: $font--role-default, $size: 1rem, $weight: 300) {
 	font-family: $family;