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`
Coding should probably follow our standard in a brand new plugin, when you think of this discussion,

https://community.elgg.org/discussion/view/1887312/consolidating-duplication?offset=10

4. Normalize plugin code.