Improve install button layout on narrow screens

This allows the button container to add line breaks between the
buttons when necessary.

snikket_web/scss/invite.scss

@@ -54,6 +54,8 @@ div.install-buttons {
 	ul {
 		display: flex;
 		flex-direction: row;
+		flex-wrap: wrap;
+		justify-content: center;
 		list-style-type: none;
 		margin: $w-l1 0;
 		padding: 0;