Extension persistence logic breaks when extension versions are removed from SMD image

## Issue
There is no impact to new spaces. This is related to extension persistence impacting existing spaces that are updated to use new SMD image.
Example scenario to show how existing spaces can be impacted:
Let's say a CodeEditor app - appA - is created with SMD 1.0 which has toolkit v1.0
And SMD 2.0 updates the toolkit extension to v2.0

If the appA is stopped and restarted with SMD 2.0 - it will not show the toolkit extension as installed anymore.
They will see a popup that says that the image has extension with a version different than in space's storage.

## Root cause
When the CodeEditor app is starting up, we "persist" the extensions from image
This is done by creating extensions folder in space EBS storage and creating symlinks for each extension from EBS storage to image. [Code link](https://github.com/aws/sagemaker-distribution/blob/main/template/v3/dirs/usr/local/bin/start-sagemaker-ui-code-editor#L50)
This means that the complete extension folder for toolkit v1.0 is not copied over and only a symlink is created
Now, when we launch the app with SMD 2.0, that folder for toolkit v1.0 does not exist anymore and we do not see that extension as installed