This change makes major updates to the comments hint structure that gets

included in the diff viewer page data. The major driver for this is that

the old implementation did not include correct information about

comments that were made against commit ranges. It was also doing a bunch

of extra queries, using the database to generate the comments

information when we already had most of what we needed fetched.
This new implementation simplifies things a little bit by compting more

information at the ReviewsDiffViewerView level, passing it down to the

DiffViewerView, instead of letting the superclass do things and then

pull the data back out of the context structure.
This makes a couple other small improvements:

The social page info definitions have been consolidated into the base

  ReviewRequestContext, and arguments added to

make_review_request_context to set them there.
The way that diff commits and other objects are queried has been

  simplified so we don't fetch things more than once.

This does not yet add UI for commit-specific comments.

Ran unit tests.
Tested all the different diff viewer views, including single diffs,

  interdiffs, and commit diffs.
Tested that the commit list and commit history diffs worked correctly.
Tested that the comments hint (in a future change) correctly listed

  all diffs, interdiffs, and commit range comments included in the

  pending review.