CON-1772: combine AssetsManager JS requests

@Wikia/platform-team / @kvas-damian 

Thanks to ``WikiaSkin::getScriptsWithCombinedGroups`` we're 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 + #5121 fix cherry-picked
* throw ``AssetsManagerException`` when not existing group is requested (i.e. fail early)
* improve logging of ``WikiaException`` exceptions
Following function is pretty long and does more than one operation. What do you think about splitting this logic into couple of separate helper functions which will be called here?