Make OIDC returnToUrl consistent for password and client certificate

[RubenVerborgh]

#648 implements a workaround for the OIDC returnToUrl in the case of WebID-TLS, pending a fix for nodeSolidServer/oidc-auth-manager#17.

Once nodeSolidServer/oidc-auth-manager#17 is closed, we should:

• verify that passing returnToUrl to oidc-auth-manager works well
• verify that oidc-auth-manager redirects to returnToUrl (preserving the hash Add current hash to redirect #562 ) in the case of username/password login
• verify that oidc-auth-manager redirects to returnToUrl (preserving the hash Add current hash to redirect #562) in the case of client certificate login