Add sort functionality to MultiLinkField

[GuySartorelli]

The MultiLinkField can't be sorted right now - we should add that functionality.

Acceptance Criteria

• Content authors can click-and-drag links in the MultiLinkField to determine their sort order
• The default sort order for Link is the sort field (meaning links are automatically in the correct order in the front-end)
• Sort order is saved on click/drag.
• JS sorting library is chosen with care towards using a well maintained library and minimise our maintenance burden.
• UX team is consultated on the design.

Notes

• The AnyField implementation provided this functionality, so we might be able to pull some of this from there.

PR

• ENH MutliLinkField sorting #166

[maxime-rainville]

That's what the AnyField UI looks like right now. The handle comes from elemental.

[GuySartorelli]

PR merged