Unify changeset line ID rendering and bring it to unified diffs

Summary:
Ref T2009. Currently, lines don't get their "C123NL456" IDs set in the unified view. This is the major way that inlines are glued to changesets.

Simplify this rendering and bring it into the HTML renderer, then use it in the OneUp renderer.

Test Plan:

• Interacted with side-by-side inlines (hovered, added, edited, deleted), saw unchanged behavior.
• Interacted with unified inlines. They still don't work, but the error that breaks them is deeper in the stack.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11983