US-14 Merchant Invoices Index

[Sykogst]

Things off the top of my head to look at

1. I made a change to the resources in the routes file, instead of param: :merchant_id, I changed it to param: :id. The route was looking funky :merchant_merchant_id, and this got rid of that, did we want it that way?
2. I added some relationships to the factories and a relationship tying unit_cost from items into invoice_item.
3. The method I used to query invoices probably should be looked over, I put it in the Invoice model as it was simpler here, but I might be missing something.
4. I have done zero formatting with the views to match the wireframes, let me know if I should add anything more or if we should revisit later.

[Sykogst]

merging into feat/merchant-invoices can be reviewed there