Rewrite display layer

[stochaztic]

Display layer as inherited from upstream is currently a jQuery mess, and the HTML can't be taken out of quirks mode without destroying the layout.

A rewrite in React or other reactive framework would fit the external-data-driven nature of the sheet as well.

Additional goals for a rewrite:

• Unification between the standard and plain styles currently supported
• Better indication of room admin, editor, or non-privileged user status
• Clearer editing controls--current controls are often missed entirely
• Drag-and-drop support for rearranging display configuration

[stochaztic]

This has begun, with the items being rewritten in Vue. Still todo: the item-picker and the map, then general refactoring.