Set a default URL for redirection after OAuth authentication

Since we do not use a SecurityFilter, the previously visited page is not
saved in the session.

src/main/java/de/thm/arsnova/config/SecurityConfig.java

@@ -144,6 +144,7 @@ public class SecurityConfig extends WebSecurityConfigurerAdapter {
 		if (facebookEnabled || googleEnabled || twitterEnabled) {
 			CallbackFilter callbackFilter = new CallbackFilter(oauthConfig());
 			callbackFilter.setSuffix(OAUTH_CALLBACK_PATH_SUFFIX);
+			callbackFilter.setDefaultUrl(rootUrl + apiPath + "/");
 			http.addFilterAfter(callbackFilter, CasAuthenticationFilter.class);
 		}
 	}