add a way to self-delete and self-create email in mailcow ui

Summary

add a way for administrators of the mailcow instance to open it up for the public, and also add a way for people to self-delete the same mailboxes

Motivation

self delete : it's annoying to create an external interface for self-deletion of mailboxes in mailcow, so it should be a part of the mailcow UI, rather than forcing people to create custom extensions for it, with self-delete you have to either add custom accounts to the interface or do smtp authentication - both having major downsides,, though, i think this should be a selectable option while creating a mailbox as there might be exceptions where you don't want users to be able to self-delete a mailbox

self create : in this case, it's fairly easy to throw together a ui wrapping around the mailcow create mailbox api, but i feel like this should be also a part of the mailcow ui,, i don't think it should be total opening to prevent spam, it should have a wait time and / or a captcha, also possibly a blacklist of what localparts you cannot have

Additional context

currently i'm thinking about creating a ui for both of these, but i find the smtp authentication thing a super annoying idea so i'm thinking of just adding either custom accounts, or leaving out the self-delete part and just leaving the signup thing there

mailcow / mailcow-dockerized