prose diff for changes in episodes

[regular]

If a newer version of an episode is loaded, display changes to last read version as a github-style prose diff.
Maybe we can link to a gh compare URL directly or use the gh API?

• research most easy way to display prose diff
• display it
• add 'I have seen the changes' button
• if clicked, update 'read_version' in model
• remove blue circle
• add or remove items from inventory