[development] Move multiple theme per site support to contrib module

[Robert Douglass]

Adrian Rossouw wrote:
>
> How many people actually use user specified themes on their sites? 
> This is the kind of functionality that only
> really clutters up the default interface, and confuses a lot of people 
> for no good reason.
>
> We should simply remove the 'enabled' checkbox, and rename the default 
> radio to 'enable'.
On the contrary, I greatly dislike the feature and pushed for the patch 
that made it a permission that had to be granted. No websites that I 
have built, and none that I visit regularly, let users choose their 
theme. 1++ for moving it into contrib.

-Robert