Not unmounting WebViews; heavy memory and computation leaks when closing a webview

[tjcouch-sil]

Describe the bug
When you close a webview, all of that webview code continues running until you shut down or reload the renderer.

To Reproduce
Steps to reproduce the behavior:

1. Open the Resource Viewer (only have one open for this test)
2. Change the verse ref
3. Notice the latest console log Resource Viewer project ID: ... was added when you changed the verse ref
4. Close the Resource Viewer tab
5. Change the verse ref
6. Unexpectedly, notice that there's at least one more log Resource Viewer project ID: .... This should not happen anymore if this is fixed.
7. Reload (Ctrl+R)
8. Notice there are no more Resource Viewer project ID: ... logs
9. Change the verse ref
10. Again, notice there are no Resource Viewer project ID: ... logs

Note: this issue is not present in built-in tabs that do not use webviews.

Expected behavior
WebView React roots should be unmounted with root.unmount() when they are destroyed. We may be able to use the onbeforeunload event inside the iframe somewhere.

Fixing this may fix #270!