Fixtures for AssetCategory

This change removes id serial column in AssetCategory and adds new primary key there. It was named slug and it is contained columns: type, parent and name. It is a natural, complex PK.
Besides I removed unique constraint on column name. It wasn't 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.