REST module handles localization by parsing lang parameter from the incoming request. It incorrectly translates _ to - for IETF based language tags.

Since we introduced full support of IETF in Magnolia (MAGNOLIA-8709), we need to adjust how locales are parsed in REST module. I18nContentSupport introduces new API for determining locale from string and so this API needs to be used in REST module to correctly parse lang param.