Working prototype

This commit implements the oidc compatibility with the caveat of having
every subsite under the app path. For that, there is also a redirection
handler to redirect the home page to the app home page.

src/main/resources/application-prod.properties

@@ -17,4 +17,4 @@ spring.security.oauth2.client.registration.oidc.scope=openid,profile,email
 spring.security.oauth2.client.registration.oidc.redirect-uri={baseUrl}/login/oauth2/code/{registrationId}
 spring.security.oauth2.client.provider.oidc.issuer-uri=${OIDC_ISSUER_URI}
 
-vaadin.urlMapping=/*
+vaadin.urlMapping=/app/*