feat: implement proper registration process

[DecDuck]

Requirements:

1. First time admin creation (detect with no users in database)
2. Invitation-based account creation, with support for pre-filling userdata. Will eventually need to integrate with admin UI

[alessiodam]

Yes, I was thinking about making it invite only on self hosted servers.

[DecDuck]

Automatic provision by SSO is also a good feature to add

[alessiodam]

Yeah but why not first get the basic stuff working :P

[DecDuck]

Yeah of course, priorities are important. I was just mentioning it for later down the line.