Sign in with Google or any generic OIDC provider

The login flow now supports multiple OIDC providers simultaneously via Ueberauth and ueberauth_oidcc. Google is built in, and a generic provider can be configured by supplying its issuer URL so endpoints are discovered automatically. Set HIVE_GOOGLE_* or HIVE_OIDC_* variables, and optionally restrict access with HIVE_GOOGLE_ALLOWED_DOMAINS or HIVE_OIDC_ALLOWED_DOMAINS. Google gets a hosted-domain hint when a single allowed domain is set. The login page shows one button per enabled provider and works on public instances too.