Increase the entropy of our generated passwords.

62 possible characters (26 upper + 26 lower + 10 digits)
If 16 characters long, 62^16 = 95.27 bits of entropy
20 characters, is 119.08 bits of entropy
22 gets us to over 128 bits with 130.99 bits of entropy, but to keep it
chunkable in groups of four, let's bump up to 24.
24 characters gets us 142.90 bits of entropy.

We have achieved [boring
crypto](http://cr.yp.to/talks/2015.10.05/slides-djb-20151005-a4.pdf)

Props @voodooKobra (for the advice on sufficient levels of entropy),
@TimothyBJacobs (for the crypto nag)

class.application-passwords.php

@@ -18,7 +18,7 @@ class Application_Passwords {
 	 * The length of generated application passwords.
 	 * @type integer
 	 */
-	const PW_LENGTH = 16;
+	const PW_LENGTH = 24;
 
 	/**
 	 * Add various hooks.