Unable to masquerade when logging in via SAML

Setting back to needs work since the patch from #21 is conflicting with #2975184: Masquerading as user not authenticated causes logout → . I think the right way forward for this issue is probably to follow the approach proposed by @berdir (co-maintainer of this module) in comment #13, and making sure we address the fatal mentioned in #14.

Added a new MR with the suggestions from #2124117-13: Unable to masquerade when logging in via SAML → and #2124117-18: Unable to masquerade when logging in via SAML → . This is meant to be used together with #2975184: Masquerading as user not authenticated causes logout → .

Looks good to me :) Thanks Pieter!