[Enhancement] The price drop percentage should be displayed with decimals in order to have an accurate discount

[Softvision-RemusDranca]

[Notes]:

• On the seller website, the discount of a product is not displayed with decimals. If you calculate the original price with the displayed discount you get a different price for the reduced product, this price can be up or down from the displayed price.
  So, if we display the price drop with decimals we show the users an accurate discount and will give them a better perspective on the products they follow.

[Affected versions]:

• Firefox Beta 63 and above

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest version of the "Firefox shopping" add-on (4.0.0) downloaded from https://testpilot.firefox.com/files/shopping-testpilot@mozilla.org/latest.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to "http://www.mkelly.me/fake-product-page/".
2. Click the "Tracked Products" toolbar button.
3. Click "Add This Product" button.
4. Refresh the "Fake Product" page.
5. Click again on the "Tracked Products" toolbar button and observe the price percentage displayed.

[Expected result]:

• The percentage is displayed with decimals.

[Actual result]:

• The percentage is not displayed with decimals.

[Additional Notes]:

• Attached is a screen recording of the issue: