Adds a branch (a local yard or store) to a supplier. name is required; address, phone,
account_number and active are optional.

You can load the branch's price list in the same request with supplier_products_attributes, an
array of entries with product_id, cost, item_code and unit. Entries without a product_id
are skipped, and each product can appear only once per branch.

The token's role must be owner or admin.