Issue #3415218 by codebymikey: Normalize the gutenberg.settings config

Whilst ensuring default values are only applied if the configuration isn't set. It includes:

• Normalization support for image styles - #3340209
• Fixing a bug where the "All" checkbox wasn't working.
• Addresses knock-on #3471570 issue with regards to providing a viable migration path without breaking existing sites.
  • It keeps the default behaviour of showing all the image styles unless the site builder specifically opts in to restrict it to specific ones. The current default approach is a bit limited in the sense that whenever you add new image styles, you'll have to manually update the image style setting for each entity bundle to include new ones.
• #3415218: Provide image styles alter hook to filter the available global styles dynamically.

If this lands in 2.x, the fixes will need to be cherry-picked onto 3.x