Implement form_post response mode#337

Harm-r wants to merge 20 commits into

simplesamlphp:wip-version-7simplesamlphp/simplesamlphp-module-oidc:wip-version-7from

Harm-r:wip-version-7-form-postHarm-r/simplesamlphp-module-oidc:wip-version-7-form-postCopy head branch name to clipboard