[ux] Pre-loading of next annotation item

**Problem**
When providing annotations through the Label Studio web-interface, it is quite disruptive to wait on the interface to save the selected labels, load and show the next annotation item.

**Posible solution**
Pre-load the next annotation item in the background, reducing the time between annotations, and time needed to load the next annotation item. Increasing labeling contributor efficiency and flow, reducing effort needed to label vast amounts of data.

**Additional context**
When pre loading annotation items, make sure their status is kept at unseen, unlabeled when the user never progresses to the next annotation item. Make sure pre-loading is limited in order to reduce load on server.