Add catalog model and seed

Catalog, or ProductCategory (will be renamed) model represents
hierarchical grouping of products. This base data, without mapping to
products is taken from the MH db.

In addition, seeds are now split into files in the db/seeds directory.

Signed-off-by: Zeljko <zeljko@zwr.fi>
Remove unneccessary comment

Signed-off-by: Zeljko <zeljko@zwr.fi>