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

TODOs:
 - [ ] Move `css/tagcloud/site.php` to `elgg/tagcloud.css`
then maybe we should change the class name? to `.tagcloud`... as the elgg- prefix is not logical there... it was just a leftover from the core css (that was moved to this plugin)