refactor: clean up cdn regex that mobile app buttons refer to

[darcywong00]

          This doesn't look right to me? But I see that it is, wow, that's pretty magical and horrible:

keyman.com/_includes/includes/template.php

Lines 127 to 130 in a2c22da

	if(isset($args['button'])){
	// Regex to see if image is from CDN
	$pattern = '/src="cdn([^"]*)"/i';
	if(preg_match( $pattern, $args['button'], $matches )) {

Definite candidate for rewrite in the future!

Originally posted by @mcdurdin in #570 (comment)