Allow project-specific descriptions of changes

[sminnee]

Right now the only way that the detail of changes are visualised is with a big read-only replica of the edit form.

I can understand why this suits the general case – it's too hard to make something that magically adapts itself to every situation – but on a specific project, I might want to write some custom code that helped render a shorter summary of the change.

For example, if I have a status change field I might want to highlight "Status: 'Open' -> 'Closed'" even in short summaries of the change.

It would be nice if there was a way to hook this kind of thing into the Activity field of the history viewer tab/control, and in other places where you use the snapshot data model.