Cache comment issue counts on ReviewRequest.

ReviewRequest now has three new counters for tracking the number of
issue states across all comments made on the review request.

These default to None for all existing review requests. When a review
request is accessed, all its counter fields will be initialized in one
go. Any time the state of an issue on a public review is changed,
the counter will be updated.

This will be used in future changes for the API and for the dashboard.
Bump the Djblets requirement to 0.8alpha3.dev.
Fixed JavaScript unit tests for file attachment thumbnails.

The file attachment HTML changed a bit, breaking some assumptions in the
unit tests. This updates the unit tests for these changes.
Concatenate raw diffs as bytes.

The raw diff method was concatenating all the individual file diffs using '',
but those file diffs may be undecoded string data that doesn't parse as
unicode. This changes it to use b''.

Trivial change.