Replace `tokens` with `assets`

[ETeissonniere]

Seems like future versions of substrate will have a dedicated trait to work with the assets pallet. We should be able to replace our custom implementation with this.

The main question would be if it is still possible to have dorgs with the native currency token.

• Swap tokens to assets in runtime
• Re-add balances pallet for native currency handling
• Find a way to still make organizations with either of balances or assets
• Update the front-end apps demo