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`
not against changing it to css, but why is the path `css/elgg/tagcloud.css` better than `css/tagcloud/site.css`

my choice allows for a better differentiation between admin and frontend specific parts. Having the word Elgg in the path makes no sense to me