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`
@jdalsem it absolutely must have a prefix. That I definitely will not compromise on ;). It is still an "official" elgg plugin, therefore the elgg-* prefix makes sense.