Manage users from a user table

[vviers]

Following up on https://github.com/gristlabs/grist-core/issues/324#issuecomment-1411730736

Say we have a Users table with custom attributes (used for granular ACLs), it would be great if adding/deleting a user in this table also added the user to the document.

As of today, the workflow for adding a new user to the document looks like the following :

1. add the user to the document (via the "Manage Users" modal) with the correct role
2. go to the Users table, create a new line with the user's email address and their attributes

It would be great if we only had to do 2. and 1. was handled automagically (though this begs the non-trivial question of which role to add the user with by default)