Reject OmniAuth sign in when email is too long

What does this MR do and why?

If an email passed via OmniAuth is longer than 254 characters, reject the sign in attempt and do not create/update the user. Normalizing extremely long strings can be intensive and email addresses should not be longer than 254 characters per spec.

Of note, we do not limit email address length in other areas of the application. We may want to consider that in the future. I queried Gitlab.com to see if we had any email addresses longer than 254 characters. It appears we do not, which is great news. It means we should be extra safe from this change though it only affects new users.

Query: Email.where('length(email)>254').count => 0

See related issue for more details: https://gitlab.com/gitlab-org/gitlab/-/issues/460714

Feature flag rollout: #475077 (closed)

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How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:omniauth_validate_email_length)

2. Follow instructions in the issue