Make pkce optional

[julien-nc]

closes #735

Some providers like Entrust refuse to show the login page and return those to the code endpoint when PKCE parameters are passed in the login page URL: {"error":"invalid_request","error_description":"Invalid request"}.

This disables PKCE by default. It can be enabled in config.php

[nc-fkl]

looks good to me

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[Kranzes]

PKCE should be enabled by default, PKCE is the right way to do OpenID Connect according to the spec. Most Idp's support PKCE I don't think it makes sense to disable PKCE by default because of one flawed ldp. The recent update broke the login functionality for most people I assume.