Universalise Number generation and usage

Finally have a standard solution for number generation.

We have `Number::type` which is the type of the sequence, be it order for now, but user, invoice, etc in the future.

Let's distinct 2 things:
- sequence number: this is an integer starting from 0. Once we want to book a sequence number, in the backend I just insert a new Number and it books it.
- order|user|etc number: this is totally customizable. You have the generator that takes the sequence number in argument, and it can generate everything (even random number). For now it just pads the sequence number, and add a start number.

I deleted the relation between Order and Number as it was useless. Once the number is generated, we don't care about what Number served for the number generation for one Order.

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