[6.x] Add a "compact mode" for the array fieldtype

[jaygeorge]

Description of the Problem

The array fieldtype could do with a "compact mode" when being used within a grid.
This is handy when we don't need all the details up front – like for custom attributes in a sites table here

These details could be tucked away:

What this PR Does

• Adds a compact mode to the array fieldtype. This is an opt-in (compact: true), and we're only using it for /cp/sites at the moment
• The compact mode includes a close button to smooth over the UX, so it’s obvious how to dismiss the modal. Esc still works
• I've also changed the behaviour of the "enter" button inside an array, so that hitting enter in the middle of the array adds a row below rather than to the end. This would be useful if you'd like to insert missing detail to the array.
• Adds a z-index helper for confirmation modals. This was needed so the confirmation modal stays above the compact array popover

How to Reproduce

1. You can see this in effect at /cp/sites in the "Custom Attributes" column