New endpoint MoveSelection

Refs https://gitlab.com/arkindex/requests/-/issues/621

We need a new endpoint named MoveSelection that asks an API user for:

• a corpus_id : this is the corpus with selected elements
• a destination : this is the element where all selected elements will be moved on

The destination must be set in corpus_id, otherwise the endpoint raises an explicit 400 error.

You must reuse the underlying asynchronous code from move_element tasks, but can declare a new trigger + task (named move_selection).