UI improvements: more buttons, keyboard shortcuts

[lgaborini]

Hello, I added some UI enhancements:

• the "Skip" button (does not update the status of the pair)
• the "Unknown" button (resets the status of the pair to neither match nor distinct)
• navigation buttons to scroll through all candidate pairs (first, back, next, last, go to first unknown)
• keyboard shortcuts associated to every action
• a div showing the status of each pair
• feedback using toasts
• a "Back" button when the end is reached

A live version is hosted here.

On my wishlist:

• make AnnotationWrapper export a JSON with similarities and matching statuses, to refine classifier outputs
• use standard colors for match/unknown/distinct elements (buttons, statuses, toasts)
• undo/redo and action history (I'm not sure it's feasible without moving the whole appData into the React state)
• add an overview screen of all candidates, to jump around by clicking (a grid view perhaps?)
• controls to filter by similarity range

(It's my first time ever using React: feel free to drop or correct my code!)