content_id must be changed when the content of a ContentNode changes

[rtibbles]

Based on feedback here:
Originally posted by @jayoshih in #970 (comment)

It seems that we may not be generating a new content_id when the content of an exercise gets changed from the ContentNode that it was copied from. This is necessary, as otherwise we will have identified these nodes as identical in terms of their content, even though their content is not the same.

[rtibbles]

I realize that there is more going on here too - we also need to change the content_id when a non-exercise content node has its default files removed and replaced with another one through the interface.

[vkWeb]

Assigning this to myself. Me and Blaine did some brainstorming on this. Will fix this one after solving user soft delete issue.