Add the ability to change the Skin & ColorTheme globally

The idea is to have an UI from the Main Wiki Administration where you can change both the Skin & the ColorTheme for all of the existing subwikis at once.

Currently you can use "xwiki:XWiki.DefaultSkin" as the skin for a subwiki and make your changes in the main wiki, but you have to manually add this value for each subwiki.

For ColorThemes there's no option to set it at once for multiple subwikis as ColorThemes are local for each subwiki.

Issue suggested by ludovic.