Prepare 8.10.0

[julien-nc]

Added

• Add debug log when storing state in PHP session during code flow @julien-nc #1412

Changed

• Optimize user counting @CarlSchwan @solracsf #1418
• Modernize user backend @CarlSchwan @solracsf #1419
• Harden UserBackend::provisionUser and UserBackend::checkFirstLogin @CarlSchwan @solracsf #1420

Fixed

• Only redirect to login flow when request comes from a navigation context, add support for storing multiple states and clean up flow session values on failure @julien-nc #1410