Support for copying and editing coded posts

[tanius]

Based on use cases like #207 and similar ones that will appear in the future, at some time we'll want support for copying and editing coded posts. It could look like this:

• Hooking into the Discourse code for moving posts to a different topic, and moving the annotations as well when that is done.
• Installing a Discourse plugin for copying a post, and hooking its code to also copy the annotations.
• Automatic adjustment ("re-anchoring") of annotations when editing coded posts.

Together, this allows splitting existing posts by copying them and editing the copies. No true copy&paste incl. annotations would have to be implemented, which would be much more complex to do.