Adding Registration reason for accounts needing approval

Hey

as of recently I have enabled the "Account approval required" feature in AdminFE. And while browsing AdminFE at the same time, I have noticed that the field "Registration reason" is mentioned a few times like in "Settings > Instance" and in "Users" when applying the "Pending approval" filter on the list.

Yet on the registration form in PleromaFE it does not appear after enabling Account approval. I believe it might be helpful to add it and maybe add it as well to the mail sent every time there is a new registration to review for themed instances to be able to select their users (or just instances that want a certain level of moderation). I think that currently the only equivalent of a registration reason would be asking users to use a temporary bio when registering their account.

That's all :)

Also, just in case, I'm on 2.2.0 on stable