Session aware logout, backend logout url approach (#1876)

* Session aware logout, backend logout url approach

* Add CHANGELOG.md and documentation for #1876

* Proper http handling and case change for golint compliance

* Update alpha_config.md

* Fix case conformity

* Change placeholder from ${id_token} to {id_token}

As this should be specified in a URL and curly braces should be escaped as %7b and %7d, therefore using {} shouldn't be an issue

* Apply suggestions from code review

Co-authored-by: Jan Larwig <jan@larwig.com>

* Add other suggestions

* Add suggestions and move background logout to generic provider

* Changelog updated

* Update oauthproxy.go

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

* Add comment for gosec, remove sensitive data from log

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Co-authored-by: Jan Larwig <jan@larwig.com>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>