Move an element

Depends on backend#731 (closed)

A new action named "Move this element" should be available when browsing elements.

Once clicked, a new modal with a Folderpicker should open, alongside a big Move XXX button (where XXX is the name of the element).

Once a folder is selected, and the Move XXX button is pressed, a call to MoveElement is triggered, and an async job is created.

The element should be immediately removed from the store (like it's done on deletion), even though it's not really moved by the backend yet.

The modal should have a light warning:

Moving an element will also move all its children element and all attached transcriptions, entities, and classifications. All current paths will be replaced by the newly created path.