Provide better support for comparing submission content for existing entries

[engram-design]

Currently, we assume that submissions are created on brand-new entries when it comes to comparing content. We should cater for the scenario where the draft is created on an existing entry and show the comparison based on that, rather than assuming "everything" is new.

[SETU-WEB]

100% agree this is a big drawback as the majority users we have setup to use workflows are updaters rather than creators.

[alexjcollins]

+1 for adding support for comparing edited entries. Would really help with a client project we're rolling out (although Craft's field highlighting will do us for now).