VE-1592: Added native support for galleries to Parsoid

This is a "production ready" prototype/proof-of-concept.
At this moment HTMLDOM for Gallery diverge slightly (typeof) from spec (https://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec/Gallery_RDFa) but this should be considered temporary and is caused by Wikia's messy approach to galleries (multiple types of galleries - and not all of them makes sense).

https://wikia-inc.atlassian.net/browse/VE-1592