Propagate Symfony responses throughout the code (#1760)

* Migrate HTTP::redirect

* Migrate HTTP::redirectTrustedURL

* Migrate HTTP::redirectUntrustedURL

* Migrate HTTP::submitPOSTData

* Temporarily revert to make phpunit work

* Migrate logout handlers

* Remove unnecessary assertion & simplify logics

* Remove unnecessary assertion & add new one to check response type

* Remove unnecessary assertion & rationalize code

* Migrate IdPDisco

* Migrate ProcessingChain

* Fix return-type & drop unnecessary use-statement

* Address review comments

* Do not add caching headers at all

* Fix type-confusions

* Migrate saml2 bindings

* Migrate completeAuth

* Migrate handleLogout

* Migrate some more

* Migrate a whole lot more (messy)

* Migrate logout

* Migrate a whole lot more (also messy)

* Many fixes

* Raise coverage

* Psalm: Ignore exteranl adfs-module

* Replace use of _GET globals

* Migrate bindings (send)

* Migrate bindings (receive)

* Fix psalm-dev

* Fix assertion

* s/subclassOf/isInstanceOf

* Fix type-confusion in logout handlers

* Update container

* Bump saml2 + adfs

* Rename namespace