WIP: Enable email notification on SSO when set in the config

[lukasdenk]

Fixes #10882.

Fix a bug where email notifications were not enabled when the user logged in via an SSO even though in the config email.notif_for_new_users and email.enable_notifs were set to true.

Tested with keycloak.

Signed-off by: Lukas Denk, lukasdenk@web.de

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[clokep]

Note that this essentially reverts #11770, which was a partial reversion of #11675. So I think there's more to this than just changing that if-statement. See #11769 for issues which occurred with this change.

[lukasdenk]

Set to WIP since the proposed solution causes the issues referenced to in #11769.

[clokep]

@lukasdenk From your point of view -- are you waiting on some feedback from the Synapse team or do you need to investigate more what's going on? I'm wondering if the review request accurately represents the current status!