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.

src/scripts/thebelabhelper.js

 function initThebelab() {
+    let activateButton = document.getElementById("thebelab-activate-button");
+    if (activateButton.classList.contains('thebelab-active')) {
+        return;
+    }
     thebelab.bootstrap();
     // Run all thebelab cells marked as initialization code
     setTimeout(function() {
@@ -9,6 +13,5 @@ function initThebelab() {
             runButton.click();
         }
     }, 500)
-    let activateButton = document.getElementById("thebelab-activate-button");
-    activateButton.parentElement.removeChild(activateButton);
+    activateButton.classList.add('thebelab-active')
 }
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