Per second and N belts display

[narc0tiq]

Add buttons to switch to ore-per-second or number of belts (the latter is really only useful with the mining productivity included, so maybe just include it by default). Not sure if making a button for each belt is the way to go, probably an item chooser instead (or alongside)

[narc0tiq]

This could go together with #116, the GUI for changing sorting order and the unit picker will likely all end up as buttons along the top of the sites display.