Fix some issues with saving the course structure

[yscik]

This fixes a few issues caused by the blocks being updated after saving:

• IDs not being assigned to the new blocks on first save
• Posts having a newer version autosaved if the user doesn't save the post again
• Post always being displayed as dirty after save

Looks like saving the post again is seamless to the user, and this is the cleanest of the possible solutions.

Changes proposed in this Pull Request

• Add check to only update the blocks if the structure is different
• Add check to only save the structure if it changed
• Save the post again when the blocks were updated after the structure was saved ( = IDs were assigned to new lessons/modules)

Testing instructions

• Create course with outline block, add lessons, customize their colors
• Publish the post
• Post should be marked as saved (course meta boxes might interfere with this one)
• IDs should be saved in the post_content for the lesson blocks
• After publishing the lessons, their style on the frontend should also look like as set in the editor
  --
• Reloading a course post after saving should not show a Newer autosaved version is available message.

[renatho]

It's looking good! =)

Just testing, I saw that in the first save it wasn't calling the resave.

[jom]

Nice solution!

It also gracefully recovers if I try to create a course, add a module, don't add a module title, add lessons, attempt to save draft (no course structure saved), and refresh. I can then resume where I left off.