Improve documentation and modernize ModificationTimestampField.
Review Request #9754 — Created March 7, 2018 and submitted
ModificationTimestampFieldhas been around since the very early days
of Djblets, and was in need of updates and better documentation. This
change adds that, removing some unnecessary logic and functions, making
better use of the parent class, and adding unit tests.
Unit tests pass.
Built the docs and checked for errors.