Often times an invoice contains multiple line items, as an example, let's say our invoice contains 1 year of hosting, 1 year of domain, and a 1 year SSL cert. The customer doesn't have enough money to pay the whole thing, but their SSL cert is expiring today so they want to pay for the domain and the SSL cert now, and pay for the hosting next week.

Invoice split would be available on edit, if the invoice has more than 1 line item. There might be a better way to do it, but I'm thinking we could do it in one of two ways.

1. Add an option to line items called "Split", so they'll have "Add, Delete, Split". Selecting Split would open a modal explaining that all items below this item, including this item, will be moved to a new invoice.
2. Another option would be to add a new column after tax called "Split" with a checkbox in it. When any items are selected, a button would appear to split the invoices. You could check off all the items you want to split, and then click the button to split them.

I like option 2 better, though I'm not sure how I feel about adding another column. A modal in both cases would be ideal to confirm the split, a split cannot be undone (unless we add a merge option later).

When selected items are split, those items will be added to a new invoice that has the same date billed, date due, currency, auto-debit status, notes, and invoice delivery options as the one it is split from.