Make monitored area (cell output) full-width and add outline around canvas

[AlphaRLee]

Currently, students can't do keyboard actions when their mouse is off-canvas and students can't see the edge of the canvas easily if they choose a white canvas (default)

AC:

• Set monitored output full-width of cell output
• Add 1 px black outline around the canvas
• Add a small green dot in the top right indicating that the mouse is inside the monitored output. Dot should disappear when mouse is outside.
• Unit test
  • Keyboard input should work when mouse is beside the canvas horizontally
  • Keyboard input should not work when mouse is above the canvas (e.g. on a different cell)
• Docs are nice-to-have