Single-Sign-On for Webmail

Summary:
Return short living password as OIDC claim. Then Roundcube will use it as user password.

Note: This requires some upstream patches, so right now you can't test this with webmail.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: mollekopf, #hkccp_developers

Differential Revision: https://git.kolab.org/D4899