Numbers for content page

- Rename /extra API to /extras to keep it consistent with other APIs
- Rename /config to /extras (without ident_hash)
- Return the number of Collections and Modules for each subject in /extras

`http://localhost:6543/extras` now returns:
```
{"subjects": [{"count": {"Collection": 0, "Module": 0},
               "id": 1, "name": "Arts"}, 
              {"count": {"Collection": 0, "Module": 0},
               "id": 2, "name": "Business"}, 
              {"count": {"Collection": 0, "Module": 0},
               "id": 3, "name": "Humanities"}, 
              {"count": {"Collection": 1, "Module": 7},
               "id": 4, "name": "Mathematics and Statistics"}, 
              {"count": {"Collection": 1, "Module": 6},
               "id": 5, "name": "Science and Technology"}, 
              {"count": {"Collection": 0, "Module": 0},
               "id": 6, "name": "Social Sciences"}
             ]
}
```

Effort points: 2