Discount query improvements

[alecritson]

Based on #1103 this PR looks to optimise the query when fetching discounts. Beforehand we just loaded all available discounts, regardless of whether they were actually needed and then left it up to the individual types to figure out whether they should apply, this will be quite taxing on sites with large amounts of discounts. This PR looks to change the following:

• Removed the custom logic for checking customer groups / channels in favour of the recently added scopes.
• Added usable scope to discounts so we can query based on whether uses has exceeded max_uses if present.
• Added products scope to discounts to query based on whether the discount is limited to certain product ids.
• The Discounts::getDiscounts() method now accepts a nullable Cart model to be passed.
• If the cart exists, we only fetch discounts which have the products in the cart is conditions or no conditions at all.
• If the cart has a coupon, we only fetch discounts which match that coupon, or if the discount requires no coupon.

This PR relies on the tweak on #1108

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Name	Status	Preview	Comments	Updated (UTC)
lunar-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 6, 2023 7:28am

[glennjacobs]

@alecritson looks good, much tidier.