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