Fixtures for AssetCategory

This change remove id serial column in AssetCategoryand add new primary key there. It was named slug and it is contained columns: type, parent and name. It is natural, complex PK.
Besides I remove unique constraint on column name. It is not necessarily demanded. The names of category can be the same in different tree's level.

The file  src/ralph_assets/fixtures/initial_data.json contains initial data for AssetCategory.