Reintroduce flag for jupyter notebook diffing

Why

Jupyter Rendered Diffs are still an exploratory feature, and we removed the feature flag too soon.

What

To avoid disruption to users, we are re introducing this feature flag for a longer term

Steps to reproduce

1. On your gdk, open a commit that contains a jupyter notebook (http://gdk.test:3000/gitlab-org/gitlab-test/-/commit/5d6ed1503801ca9dc28e95eeb85a7cf863527aee)
2. By default, the feature flag is on, so it should display the transformed diff
3. Disable the feature flags:

   Feature.disable(:ipynb_semantic_diff)

4. Restart gdk
5. It should display the raw diff now:

Related to #358909 (closed)