Ensure that insertUsage is always present in preferences

[gziolo]

Description

Follow-up for #14691. Another fix for #14580 :)

I did some more testing with master branch and discover another case when an older version of Gutenberg explodes.

Testing instructions:

1. (Prerequisite) Update to WordPress v5.1.1
   • In other words, be running the latest version, or at least not WordPress v5.2.0-beta.1
2. Clear localStorage
   • localStorage.clear() in your Developer Tools Console
3. Navigate to Posts > Add New
   • Toggle Publish sidebar through Options modal.
4. Deactivate Gutenberg from the Plugins screen
5. Navigate to Posts > Add New
6. Click on the Inserter
   • It uses insertUsage preference from core/editor.
7. Activate Gutenberg plugin from the Plugins screen
   • Make sure the plugin is a built version of this branch.
8. Navigate to Posts > Add New
   • You don't need to take any action, just visit. The preference migration takes place automatically.
9. Deactivate Gutenberg from the Plugins screen
10. Navigate to Posts > Add New
11. Click the Inserter
12. Verify that no error occurs

[aduth]

I am approving, but at the same time I don't think we need or want this:

1. Don't need: I can't reproduce the testing instructions causing an error in the master branch, and I wouldn't think it should, since the migration doesn't ever remove the preference from the core/editor store. If the point is about it affecting earlier versions, changing it here doesn't matter since it won't help those earlier versions.
2. Don't want: We should be skeptical toward anytime we'd ask "How long do we want to keep this dead code?" and the answer would be "I dunno, I guess forever?", which is how I feel toward this. Otherwise, it becomes a burden for future maintainers (and perhaps even a source of future bugs).

As it impacts Monday's Gutenberg 5.4 RC, it can be safe to merge, and we can continue the discussion afterward, since if we later decide it won't hurt to not have the preference, we could similarly safely remove it in the future.

[gziolo]

I think that it might be also necessary to toggle on/off Publish panel between steps 3 and 4 to trigger saving editor preferences in the local storage. I will try again on Monday with screencast recording enabled, I will merge this change only if I’m able to reproduce on master.

[gziolo]

I reproduced it on master: