Rework db-seeding, so that it doesn't depend on a github auth token

Instead, admins are specified using a JSON file, 'admins.json'. This file is
gitignored. If it is not present, there is a default list of admins in
'admins.default.json'.

crates/collab/src/db/queries/users.rs

@@ -20,7 +20,11 @@ impl Database {
             })
             .on_conflict(
                 OnConflict::column(user::Column::GithubLogin)
-                    .update_column(user::Column::GithubLogin)
+                    .update_columns([
+                        user::Column::Admin,
+                        user::Column::EmailAddress,
+                        user::Column::GithubUserId,
+                    ])
                     .to_owned(),
             )
             .exec_with_returning(&*tx)