style: add utility classes, apply borderbox to all elements

src/styles.scss

@@ -39,3 +39,17 @@ html, body {
     margin: 0 0;
     height: 100%;
 }
+
+* {
+  box-sizing: border-box;
+}
+
+// Utility classes
+
+.full-width {
+  width: 100%;
+}
+
+.top-auto {
+  margin-top: auto;
+}