Reduce code duplication in inline right/left side tracking
Summary: Ref T2009. These subclasses have a mixture of similar methods, move them all to the base class.
Test Plan: Created/edited/undo/submitted comments on the left and right sides of a diff.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T2009
Differential Revision: https://secure.phabricator.com/D12024