Add live code editing using thebelab

This MR adds the ability for live code editing.

• Code that is not allowed to be edited should be marked using {python editable=false}. A button under this cell will appear that enables editing for all cells.
• Code that should run on initialization should be marked using {python initialize=true}. This code is automatically run when the kernel is connected.

Todo:

• Wait until 2 PRs are merged into thebelab so we can use them.