Add release notes for Review Board 2.5.8.
Review Request #387 - Created Feb. 10, 2017 and updated
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. Testing Done: Built the docs and read through them, checking links. Checked for spelling errors. Reviewed at https://reviews.reviewboard.org/r/8727/