Prevent modification of contentnode extra_fields during publish

[rtibbles]

Summary

Description of the change(s) you made

• When the thumbnail encoding is not set on a content node at publish time, it gets generated from its thumbnail file, and set on the node. This node is then saved.
• Adding the extra_field migration standardization has caused issues, because it modifies the extra_fields dict during the transformation, so when it got saved with a new thumbnail encoding these updates also got saved
• This fixes this issue by restricting the save to only the thumbnail_encoding field
• Adds regression tests for both legacy and new style exercises

Fixes#3799

[pcenov]

I'm no longer able to replicate the issue described in #3799 and modifying the completion criteria of an exercise + saving or editing is functioning correctly now.