[Avatars] Calculate 'sane' hue precissions

We used to get the numeric value of the entrire md5 string which is a
128bit integer. We would then devide this by the maxval of a 128bit int.

There is no need for such huge computations. As we just require a value
between 0 and 255. Thus using two 16 bit values is more than enough to
get the precision we need. By just taking the MSB we get nearly
identical results.

core/js/placeholder.js

@@ -51,8 +51,8 @@
 		// set optional argument "text" to value of "seed" if undefined
 		text = text || seed;
 
-		var hash = md5(seed),
-			maxRange = parseInt('ffffffffffffffffffffffffffffffff', 16),
+		var hash = md5(seed).substring(0, 4),
+			maxRange = parseInt('ffff', 16),
 			hue = parseInt(hash, 16) / maxRange * 256,
 			height = this.height() || size || 32;
 		this.css('background-color', 'hsl(' + hue + ', 90%, 65%)');