Ensure values are floats before doing calculations #8183

[ashleyfae]

Fixes #8183

Proposed Changes:

1. Convert values to floats before doing calculations.

Note: For ease of testing, I'm doing this PR in two parts. This first part only fixes the calculations. This is to ensure you can still save a non-numeric discount for testing purposes. After this has been tested, I'll follow up with sanitizing amounts before saving.

To test:

1. Create a new discount code. Type = percentage; amount = 50b. (Or any random letter thrown in.) Save.
2. After saving, it will appear in the UI as just 50, but in the database it's actually saved as 50b.
3. Add an item to your cart, proceed to checkout, and apply this discount code.
4. Ensure you don't get any PHP notices.
5. Ensure discount amount is correctly displayed on checkout.

[robincornett]

Confirmed that an improperly formatted discount amount is saved to the database, but the calculation on checkout matches what the discount appears to be.

[spencerfinnell]

Confirmed a percentage discount saves as 50b but still applies 50% off at Checkout.

[LisaCee]

I'm not sure what table the discounts live in, but I was able to create a non-numeric discount code and use it with no PHP errors.