R1:193a9611e40a

Partially generalize Remarkup previews and add support to Differential

Summary:
Ref T3671. A lot of applications have pretty ad-hoc preview code. Clean it up a bit and add Summary preview to Differential.

After ApplicationTransactions we might want to try to serialize the whole form and show a preview of all the transactions, but this seems not very useful in most cases (I'd guess that Remarkup previews are 99% of the value) and tricky to get right (e.g., adding images which don't exist yet to Pholio…