Added back channel logout endpoint without context

[shepilov]

The previous design had three practical problems:

• The provider had to know stack’s internal OIDC context name to call back channel logout.
• When sid was available, stack could still end up logging out more than the current OIDC session

With this PR:

• Cozy now supports POST /oidc/logout in addition to the existing POST /oidc/:context/logout
• context is resolved server-side from the logout token and existing sid bindings
• local browser sessions are bound to OIDC sid
• OIDC-derived OAuth clients are also bound to OIDC sid
• backchannel logout can revoke both bound browser sessions and bound OAuth clients
• the existing contextful endpoint remains supported