Add release notes for Review Board 2.5.8.

Review Request #8727 — Created Feb. 8, 2017 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
c43e14d...

Reviewers

This is a pretty big release with a large number of bug fixes, usability
improvements, configuring improvements, and some new features. This
change details the release notes for all these additions going into
2.5.8.

Built the docs and read through them, checking links.

Checked for spelling errors.