Provide "Restore Colour" Button

[MatthewLM]

Currently if the colour of a pixel has changed through the "I have made payment" button, it is not possible to change it back to the original colour unless the blockchain state reflects a new colour.

This issue proposes that a "Restore Colour" button is added to the payment modal for a colour that is present within the canvas state but has been changed via the "I have made payment" button. This new button would change the pixel back to this colour without needing to refresh the page.