Add release notes for Review Board 3.0.7.

Review Request #9970 — Created May 23, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
50302f1...

Reviewers

This release primary focuses on the new privacy capabilities, and
contains fixes for some repository and diff viewer issues, amongst other
bugs.

Built the docs. Checked for build errors, spelling errors, and bad links.