Fixes the OIDC return_url param when a subpath is configured

[foot]

• We should return the configured subpath NOT the root which might be another app etc.
• Also fixes the redirect_url handling for OIDC, so if you land on /sources and are redirected to signin, logging in with OIDC will also redirect you to /sources on success (and /wego/sources if a subpath it configured)

Follow up to:

• UI works on subpaths #3992
• Fix the OIDC login button when using a RoutePrefix. #4022

To test

Spin up a kind cluster w/ ingress support:

• Run ./tools/kind-with-ingress.sh
• Configure OIDC w/ the correct redirectURL

  kubectl create secret generic oidc-auth \
    --namespace default \
    --from-literal=issuerURL=<issuerURL> \
    --from-literal=clientID=<clientID> \
    --from-literal=clientSecret=<clientSecret> \
    --from-literal=redirectURL=http://localhost:30080/wego/oauth2/callback
  

• Uncomment ingress and additionalArgs sections in tools/helm-dev-values.yaml
• tilt up
• open http://localhost:30080 note the 404 from ingress controller
• open http://localhost:30080/wego/ login w/ OIDC, note we are now redirected back to /wego/ and not / (which gives a 404).

[AlinaGoaga]

Looks good 🥳 , tested and it behaves as expected :)