Enable Swagger's OAuth2 support

This PR is a part of a changeset that is required to support OAuth2-protected API. Main PR: https://github.com/metosin/ring-swagger/pull/25

Two changes are introduced by this PR:

* swagger-ui submodule is updated. This is needed because of OAuth2 redirect path construction bug that have been fixed in https://github.com/swagger-api/swagger-ui/commit/9b0aa32941473342f51526c177a48ab7f49cc4ea
* `initOAuth` is called if available and `API_CONF` provides OAuth2 config.