ViSR: Make plots size correctly - #164

This is the only solution I have found so far that achieves all 3 of these objectives:

• Plots fill their parent box
• SliceView plot resizes correctly when the controls drawer is opened and closed
• Plots resize correctly when the window size is changed

The pay off for all 3 of these working is that when the window is resized, the whole React-Grid remounts, so we see a white screen whilst the window resizes and the grid/plots re-render.

This also has added 2 'Box' components around the Heatmap plot in SliceViewer.tsx which might be less ideal for making it a reusable component.

Other solutions I have found have only achieved max 2/3 of the list.