[15.0] purchase_order_approved: button "Convert to Purchase Order" and minimun amount

[dalonsod]

Hello,

When Purchase Order Approval option is enabled with a certain minimum quantity set, this issue is raised:

• A Purchase User can only mark the PO as to approve if total amount is greater than minimum quantity (as expected).
• Once PO is confirmed by a Purchase Manager, as purchase_order_approved is installed, the PO is moved to approved state.
• Purchase User how has "Convert to Purchase Order" button visible, but clicking it... nothing happens. PO remains at approved state.

This is because, at this point, the same restrictions that apply when PO is in draft state are applied again for approved state, in order to prevent Purchase Users to totally confirm a PO with a minimum quantity. This leads to a "double approval" required to a Purchase Manager (he already approved the PO, but he needs to Convert it to PO as well, because Purchase Users can't).

Purchase Users should be enabled converting to Purchase Order when purchase is already approved IMO. What do you think? Anyway, if they shouldn't convert to PO, that button should be hidden.

If you agree with the first option (enabling Users to convert to PO), I could make the PR to fix it. Thanks!!

cc @LoisRForgeFlow @lmignon @kongrattapong