implement meta-data rendering preview for revision UI

[hvr]

The new workflow:

1. edit .cabal ("submit" button disabled on edit)
2. press "preview" button (activates "submit" button)
3. user reviews
   • new rendered preview appearance
   • warnings/errors reported
   • compute deltas
4. if needed, go back to 1; otherwise proceed to 4. (or give up & simply close browser window)
5. press "submit" button
6. revision gets published (unless revision throttle rate exceeded), and similar output as in step 3. gets rendered