Add a EMAIL_DOMAIN_SIGNUP_WHITELIST configuration option

[stdmr]

Feature Description

Our current use case is, that we restrict user registration via the EMAIL_DOMAIN_WHITELIST option. But after a successful registration each user should be able to add secondary email addresses (or change the primary email address) without any restrictions.

EMAIL_DOMAIN_WHITELIST is to restrictive! There was already a discussion about the possibility to allow admin users to circumvent the EMAIL_DOMAIN_WHITELIST (see: #28925 ), which was accepted.
But regular users still can't change their email to an address which is not on the email whitelist.

Our proposal: We want the email whitelist check only for new signups. Nothing else.
To not break workflows for other gitea instances it may be the best to just add another configuration option EMAIL_DOMAIN_SIGNUP_WHITELIST (name is just a proposal from me) which is only checked when a user tries to signup supplying an email and a password.

Screenshots

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