Classes Menu Item Property Should be array

[filosofo]

• We use it as an array almost everywhere.
• PHP array functions are pretty quick.
• This reduces some of the kludgy parts of the code and makes it consistent.
• There are a ton of classes on the public menu items that no doubt people will be filtering. Best not to make them have to explode by whitespace and then do array-handling: let's just make it arrays all the way.

[ryan]

(In [14944]) Make classes property for menus an array. Props filosofo. fixes #13551