Fix a bug causing non-eligible adjustments to appear in the the admin…

[jacobherrington]

… cart

Fixes #2484.

ES6 would make this significantly cleaner, but it seems we favor ES5 for now (happy to make that change though!)

I don't have a ton of experience with Backbone.js, so if there is a more performant built-in solution for this I'd love to hear about it.

[kennyadsl]

@jacobherrington thanks! I'm ok with the change but can you please explain better in the PR and/or in the commit message description why filtering adjustments by eligibility should solve the issue?

Also, maybe we should also add a regression spec for this, right?

[jacobherrington]

@kennyadsl essentially the frontend is displaying all adjustments including those which are ineligible on the order. They are not actually being applied to the order, just displayed on the page this JS effects.

Filtering them by eligibility causes this function to return an empty array which is what should happen when there are not adjustments affecting the order.

[kennyadsl]

Can you also please take a look at the specs for this? I'm seeing that we should already be covered but it looks like that test is a false positive.

[jacobherrington]

@kennyadsl that test is actually running on another page. That is running admin/orders/:id/adjustment (and should pass), but this change affects admin/orders/:id/cart. I will look into writing a regression spec for this change.

[ericsaupe]

@jacobherrington any update on this?

[jacobherrington]

@ericsaupe I haven't had a chance to revisit it unfortunately!