Allow magic link login even if the email is enrolled with an SSO provider

Hey folks.

At our company we are self-hosting Outline with Slack and Magic Link authentication providers enabled. We're currently migrating off of Slack, but would like for both auth methods to be available in the interim.

Our problem was that if the email was enrolled in Outline via Slack provider, even if the user selects the email authentication, they get redirected to Slack to authenticate there. Here's the code that enforces this behavior:

We have changed this in our fork of Outline, and we would be interested in bringing this change to upstream. I propose introducing a new environment variable PREFER_SSO_OVER_EMAIL_AUTH with the default value of true to keep the current behavior intact. If this is set to false, Outline would allow signing in via email even if it is enrolled with an SSO provider.

Would you be interested in accepting a pull request with this change?