CON-1487: generic support for concatenation of SASS files

https://wikia-inc.atlassian.net/browse/CON-1487

@kvas-damian / @wladekb / @Wikia/platform-team 

This PR brings a generic handling of SASS files concatenation for all skins. Currently only Oasis has this front-end performance related feature.

* in Venus this change saves ~10 requests to SASS files
* in WikiaMobile only a single request is made (instead of four) - pinging @hakubo 

Each call to ``Wikia::addAssetsToOutput`` with SASS group passed adds a new ``<link>`` tag to ``$wgOut``. This change ports Oasis-specific filtering and concatenating of SASS requests into a generic code that can be used in all skins.

@wladekb: unit tests were added :)