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.

Current usage in Elgg is mixed. It is hard for developers to guess what is standard, so please show me a design doc or be a bit more gentle...

So resume:
- prefix stays intact (Reason: because it is an elgg plugin, the plugins specific css should be prefixed `.elgg-<plugin_id>-class`
- css files should have .css extension
- file path is `/views/default/css/elgg/tagcloud.css`

To get more clarity on the latter... what does elgg mean in the file path? Is it related to which view it is extended on? Is it because it is a core plugin? 
What if i wanted to extend the admin css also from the tagcloud plugin (with a seperate view); what will the file name be then?
What would be the prefered file location for plugins? `/views/default/css/elgg/<plugin_id>.css` or ``/views/default/css/<plugin_id>/site.css`? Or something else?

If we could finish this discussion here (as this is seemingly the first new case where we add a new css file) i will move the conclusion to a seperate ticket to add this to the docs (as a plugin skeleton / css guideline).