Use a number field for cart item quantity

[mamhoff]

Summary

Selects for numbers are a bad idea. It is possible for our variants to have a count_on_hand of Infinity (by unchecking the "track inventory" checkbox. On my machine (32G memory), this took about ten minutes to fill up my memory.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.