more robust unit system

Some items have units:

```
{
"id": "36",
"name": "kevlar shock cord",
"description": "Perfect for model rocketry. Used to attach the rocket body to the parachute.",
"image": "/assets/kevlar_cord.jpg",
"price": "0.3 dUSD",
"unit": " m"
},
```

The unit string is prefixed with a space if the unit should be displayed as such. Also, currently units can only be placed after the item amount, not before (but there may be edge cases where this should be supported too).