Use sort handle for signups on trip admin tab to make it scrollable on mobile

[collinmay]

Re-addresses #105.

On mobile, the active area for starting drag-and-drop for signups on the admin spans almost the entire width of the screen, making it very difficult to scroll. By reducing the active area for starting a drag-and-drop operation to a smaller icon, it makes it much easier to scroll the page on mobile, while reducing the chance of accidentally reordering signups.

Here's a recording of what it looks like to use the page in firefox dev tools simulating a mobile device. The cursor icon bugginess is an artifact of simulating touch inputs with a mouse.

On my actual phone, the scrolling experience is much more comfortable and the active area for beginning a drag is big enough to hit when I do mean to hit it.

On desktop, the cursor only switches to the grab icon when you are hovering over the drag handle.

I've left the other usage of ui-sortable in the lottery ranking selection alone. There are some minor CSS changes, but the cursor still behaves properly there.

[DavidCain]

I like this idea. Let me try it out and I'll likely merge it. Thanks for the work here. I was really hoping I'd have completely replaced this flow by now, but alas.