DAR-1688 Store background image size in ThemeSettings if it's not set

This avoids call to wfFindFile on each call to getOasisSettings
when background image data is not set; instead, we populate
those fields so the situation will not repeat itself in future.

https://wikia-inc.atlassian.net/browse/DAR-1688
Merge pull request #1658 from Wikia/DAR-1688

DAR-1688 Store background image size in ThemeSettings if it's not set