mailu: enable central database, improve token creation task, and add migration guide

- Enabled central_database in Mailu config
- Improved API token creation task:
  * use curl -f to fail on HTTP errors
  * added explicit failed_when and changed_when conditions
- Adjusted docker-compose template spacing for readability
- Made logging level configurable (DEBUG when MODE_DEBUG is set)
- Added new documentation Move_Domain.md explaining safe procedure for migrating mailboxes to a new domain

roles/web-app-mailu/config/main.yml

@@ -7,7 +7,7 @@ features:
   css:                    false
   desktop:                true
   oidc:                   true
-  central_database:       false                   # Deactivate central database for mailu, I don't know why the database deactivation is necessary
+  central_database:       true
   logout:                 true
 server:
   domains: