BAC-1004:ThemeDesigner does not work with new Swift Storage (affects wikis with short bucket name)

@Wikia/platform-team

https://wikia-inc.atlassian.net/browse/BAC-1004

Introduce two helper methods in ``ThemeSettings`` class that return up-to-date full URLs for wiki background and wordmark, instead of returning them from ThemeDesigner settings stored as JSON in WikiFactory (which can be outdated during images migration to Swift).

Please review:

* @themech - ThemeDesigner changes
* @federico-lox, @hakubo - Mobile code changes
* @sebastian-wikia - Oasis and SassUtil changes