VE-1564 Port user options API endpoint from upstream MediaWiki
VE-1564 Use options API call to set whether user viewed dialog
VE-1564 missing semicolon
VE-1564 Fixes from code review
VE-1564 Do not let exceptions from invalid preferences bubble to API response
VE-1564 Handle non-existent and non-default preferences more gracefully
VE-1564 Changes from code review
Merge pull request #4903 from Wikia/VE-1564

VE-1564 Use options API call to set whether user viewed dialog