Allow to add and remove admins in the Settings

**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?)

