Fix Postgres database importing/upgrading conflicts

We were running into conflicts, because having initialized
the roles (users) and databases, trying to import leads to
errors (role XXX already exists, etc.).

We were previously ignoring the Synapse database (`homeserver`)
when upgrading/importing, because that one gets created by default
whenever the container starts.

For our additional databases, it's a similar situation now.
It's not created by default as soon as Postgres starts with an empty
database, but rather we create it as part of running the playbook.

So we either need to skip those role/database creation statements
while upgrading/importing, or to avoid creating the additional database
and rely on the import for that. I've gone for the former, because
it's already similar to what we were doing and it's simpler
(it lets `setup_postgres.yml` be the same in all scenarios).

group_vars/matrix_servers

@@ -1100,6 +1100,20 @@ matrix_postgres_additional_databases: |
     }] if (matrix_dimension_enabled and matrix_dimension_database_engine == 'postgres' and matrix_dimension_database_hostname == 'matrix-postgres') else [])
    }}
 
+matrix_postgres_import_roles_to_ignore: |
+  {{
+    [matrix_postgres_connection_username]
+    +
+    matrix_postgres_additional_databases|map(attribute='username')
+  }}
+
+matrix_postgres_import_databases_to_ignore: |
+  {{
+    [matrix_postgres_db_name]
+    +
+    matrix_postgres_additional_databases|map(attribute='name')
+  }}
+
 ######################################################################
 #
 # /matrix-postgres