Add ability to configure which rendering transformation to execute where

Right now, all transformations are always executed. However there are plenty of cases when you don't want all of them executed in some pages, some spaces or some wikis.

For example the icon transformation is ok in most cases but there are instances where you don't want it to execute. Same for the numbered headings transformation, and more generally for all transformations.

Thus we need a way to say either (we need both probably):

• Whitelist: That this list of transformation should execute on this page, inside this space only, inside this space and nested spaces, inside this wiki.
• Blacklist: That all transformations execute except this list of transformation ids which don't execute for this this page, for this space only, for this space and nested spaces, for this wiki.

Implementation ideas:

• Define a specific XObject that you place inside a page, inside a space home, in a special page of a wiki and that lists all tx that should or shouldn't execute.
• Have an Admin UI listing all such objects to have a global overview of all tx that execute in the wiki.
• These XObject definitions must be cached for perf reasons
• This could be used so that RenderingConfiguration.getTransformationNames() will take these XObject definition into account, see XWIKI-15100