Set mininum line item quantity in admin cart

[mamhoff]

Description
It does not make much sense to add zero of anything to an order. This
commit changes the minimum quantity for a line item in the admin cart to
one.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

I don't think a test is necessary for this change.

[kennyadsl]

This makes sense, thanks @mamhoff ! 👍

[tvdeyen]

Absolutely. Thanks 👏

[kennyadsl]

I'm giving a live try to this PR. I think it's ok as is since it does not allow you to set 0 as value by using browser's number input controls. Anyway, a user is still able to fill that input with 0 and submit. I think this happens since the submit of that form is never really called. Maybe it's just matter of triggering the validation programmatically in the backbone view?

[jacobherrington]

I think this is good as it is, of course there may be room for improvement as @kennyadsl has said.