[#652] Fix creation of new budget items in post_import.py

When creating new budget items, the script breaks when trying to create
items with the "total" label.

Fix by creating BudgetItemLabel.TOTAL_BUDGET_LABEL_ID and referring to
it rather than a fixed int in the script.
Merge pull request #653 from akvo/feature/652_post_import_budget_fix

[#652] Fix creation of new budget items in post_import.py