Admission-based registrations mode

[Gargron]

Fix #6856
Fix #6951

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Between open registrations, and closed registrations, admission-based registrations mode allows servers to accept new users in a manually controlled manner. To the outside viewer, there are no practical differences between open and admission-based registrations, as both the sign up API and the sign up form remain available. However, "Sign up" on the form changes to "Request an invite".

Accounts created when admission-based mode is active cannot be logged into or used via API tokens.

After requesting an invite, the user receives a confirmation e-mail as usual. It is communicated in the message that the account won't be available straight away, but that it is instead pending review.

Once the e-mail is confirmed, the staff receives an e-mail about a new pending account. They can then approve or reject the user's application. Rejection removes the user's record and frees up the username. Approval, on the other hand, unlocks the new account and sends the user the welcome e-mail.

Note: Because registrations_mode is technically a new setting, if you had previously closed registrations, you will need to close them again after this update! However, since admission-based mode is safe for all intents and purposes, it should be a better new choice than simply closing registrations in most cases.

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• Change database schema
• Change admin setting for registrations
• Change user-facing strings about registrations
• Change tootctl for open/close commands
• Add UI for viewing and approving pending registrations
• Add e-mail notifications about pending registrations
• Change confirmation e-mail to mention pending review

[todoneunl]

@Gargron Is there some kind of form where a new user (that asks for admission) can type the reason why they want to have an account? If not, than I think this is a must-wanted feature. Or else it would be very difficult for admins and mods to approve or decline a request.

[ghost]

Would be nice if we could only enable the email notifications for specific mods, or disable all together. On a popular instance with a team of moderators this can create thousands of emails every day. On our instance only a couple of people would be responsible for approving new signups, not the entire moderation team.