Proofing system -- tracking thread

[Arun Prasad]

This is a tracking thread for working remaining before proofing is generally available to all users.

Two big items remain:

1. Ship an updated version of the proofing interface that better handles complex content. Otherwise everyone will have to retrain on the new system, which is confusing.

2. Write a minimal tutorial about our proofing policy in light of (1).

The rationale for (1) is that we want to better support what is called "diplomatic proofreading," i.e. matching the original page content as closely as is feasible. We support this for most texts, but some texts have complex layouts that our current editor does not support (e.g. inline paragraph breaks).