Add debugging and a script for issue count calculation.

There have been some reports of issue counts failing to calculate
correctly. The cause is not obvious, so we need better debugging.

This adds some debug logging to show all the calculation state for a
given review request's issues, which users can send us when they
encounter problems.

It also adds a reset-issue-counts management command, which will reset
issue counts for particular review requests, or all review requests if
passing --all.

Reviewed at https://reviews.reviewboard.org/r/6366/
Add help output for the rb-site manage command.

This includes some basic help output for rb-site's 'manage' command that
explains how to get the list of commands and execute them.

Reviewed at https://reviews.reviewboard.org/r/6367/
Add release notes for Review Board 2.0.8.

This covers all the fixes made since 2.0.7, and adds several new
contributors to the AUTHORS list.
Update the minimum versions of our dependencies.

This adds minimum versions of Haystack and Whoosh, in order to resolve
some version issues. It also updates the minimum versions of Djblets and
Django Evolution.
Release Review Board 2.0.8.
Bump to 2.0.9 dev.
Merge branch 'release-2.0.x'