Move tagcloud out of core

Fixes #7140 

Tested and all components still work (tags page handler, sidebar tagclouds and theme preview component)

/cc @ewinslow 

Only left the language key "tagcloud" in core as it is a generic word
I put a lot of effort into making it possible to do this. I'd appreciate using the feature when we have the chance. :) It is the Right Way now, but no one knows it because we don't do it in core. And we don't do it in core because... we don't do it anywhere else in core... ;)