Add new email/confirmation flows

[HaoK]

Part of #8356

Still working on fixing/adding tests, for the default confirmation page, we generate a new confirmation link and display it only when in development.

@ajcvickers @blowdart

[HaoK]

For this new requireConfirmedAccount setting, are we planning on setting it explicitly in the templates or just turn it on in AddDefaultIdentity?

[HaoK]

Tests added/updated and are green again, removing WIP label

[javiercn]

Overall looks good.

The only real issue I have is with the way we do the confirmation link for development.

[javiercn]

Looks good to me.

[MarcAir]

There may be a logical error in the confirmation of email.

'OnPostChangeEmailAsync' inside 'Email.cshtml.cs' does not check to see if the email is used by another user prior to sending confirmation.
or should we consider this something the 'ConfirmEmail.cshtml.cs' page is to do, since someone could potentially have either errored while entering the new email, or someone trying to exploit the change email function to see if someone is registered with that email?
either way, should we not be checking for email existing in the identity store prior to emailing a request for confirmation?

something like this?
basically if email exists, do not send email. But notify user of email sent (for potential nefarious actors)

public async Task<IActionResult> OnPostChangeEmailAsync()
        {
///removed for brevity 
if (Input.NewEmail != email)
{
            var emailExists = await _userManager.FindByEmailAsync(Input.NewEmail);
            if(emailExists == null)//not notifying if email sent or not.
            {
                        var userId = await _userManager.GetUserIdAsync(user);
                        var code = await _userManager.GenerateChangeEmailTokenAsync(user, Input.NewEmail);
                        var callbackUrl = Url.Page(
                                    "/Account/ConfirmEmailChange",
                                    pageHandler: null,
                                    values: new { userId = userId, email = Input.NewEmail, code = code },
                                    protocol: Request.Scheme);

                        await _emailSender.SendEmailAsync(
                                    Input.NewEmail,
                                    "Confirm your email",
                                    $"Please confirm your account by <a href=\"{HtmlEncoder.Default.Encode(callbackUrl)}\">clicking here</a>.");
            }
            StatusMessage = "Confirmation link to change email sent. Please check your email";
            return RedirectToPage();
}
///removed for brevity 
}