Breaking model

Apply text provided by @mtbc regarding breaking changes to the 5.0 model: namely `OriginalFile.sha1` --> `OriginalFile.hash`. Since there was no clear landing page for putting the breaking info, I put some under `whatsnew` and the details with the matching documentation.
I disagree on the comma, but otherwise :+1: (btw, these are all @mtbc's text blocks, so perhaps we'll have a third voice soonish) :smile: