CON-1772: combine AM JS requests

@Wikia/platform-team / @kvas-damian 

Thanks to ``WikiaSkin::getScriptsWithCombinedGroups`` were now able to fetch all AM groups in a single request - one request instead of five.

* introduce ``WikiaSkin::getScriptsWithCombinedGroups`` for rendering bottom JavaScript tags - both external requests and inline
* load AM config only once
* throw ``AssetsManagerException`` when not existing group is requested (i.e. fail early)
* improve logging of ``WikiaException`` exceptions