Added release notes for Review Board 3.0.3.
Review Request #9592 — Created Feb. 5, 2018 and submitted — Latest diff uploaded
This covers some of the improvements and fixes made to avatars,
compatibility issues with older versions of Python 2.7 and Subvertpy,
the new File Upload avatar UI, and more.
Built the docs. Checked for build errors, spelling errors, and bad
links.