Allow to add and remove admins in the Settings

[Errorname]

Is your feature request related to a problem? Please describe.
Currently, the first user to connect to the instance will be promoted admin. After that, any changes to the admins (add or remove) need to be done manually with a graphql query.

Describe the solution you'd like
In the admin interface, I want to be able to add a new admin by typing his name (or email?)