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.

docs/configuring-playbook-bot-matrix-registration-bot.md

@@ -16,7 +16,7 @@ By default, the playbook will set use the bot with a username like this: `@bot.m
 
 (to use a different username, adjust the `matrix_bot_matrix_registration_bot_matrix_user_id_localpart` variable).
 
-You **need to register the bot user manually** before setting up the bot. You can use the playbook to [register a new user](registering-users.md):
+For [other bots supported by the playbook](configuring-playbook.md#bots), Matrix bot user accounts are created and put to use automatically. For `matrix-registration-bot`, however, this is not the case - you **need to register the bot user manually** before setting up the bot. You can use the playbook to [register a new user](registering-users.md):
 
 ```
 ansible-playbook -i inventory/hosts setup.yml --extra-vars='username=bot.matrix-registration-bot password=PASSWORD_FOR_THE_BOT admin=yes' --tags=register-user