Replace `react-selectable` dependency with something maintained

[GuySartorelli]

react-selectable looks like it may have been made specifically for asset-admin but is no longer being maintained.
It also has some strange coupling - e.g. see https://github.com/unclecheese/react-selectable/blob/802b1ec9ad1ef97fd241f48e612dd1b334b447b8/src/selectable-group.js#L126-L127

I recommend we replace it with https://www.npmjs.com/package/@air/react-drag-to-select

Acceptance Criteria

• react-selectable is replaced with a maintained alternative

PRs

• ENH Replace react-selectable with react-drag-to-select #1550
• FIX Prevent selecting pagination text silverstripe-admin#1913

[emteknetnz]

No changelog PR for this one - we have a broad "Several JavaScript dependencies have been updated, and a few have been replaced." in the 6.0.0 changelog already. I don't think this dep warrants its own mention because it's so specialized.