Add matrix-user-creator role - automatic user account creation support

We no longer ask users to create Matrix user accounts for these bots:

- Postmoogle
- Honoroit
- Reminder Bot

Other bots and services (matrix-registration-bot, maubot, mjolnir,
Dimension, etc.) require an Access Token to run (not a password),
so this new role doesn't help for them.

It does help for the above bots though, and for defining your own
"initial user accounts" in the `matrix_user_creator_users_additional`
variable.

setup.yml

@@ -72,4 +72,5 @@
     - matrix-aux
     - matrix-postgres-backup
     - matrix-backup-borg
+    - matrix-user-creator
     - matrix-common-after