Improve inline comment previews

Summary:

• When an inline comment preview corresponds to an inline comment on the page, link to it. Just punt in the tough case where the inline is on some other page.
• In "haunted" mode, "z" now toggles through three modes: normal, comment area only, and comment + previews.

Test Plan:

• Viewed visible and not-visible inline comment previews, clicked "View" links.
• Tapped "z" a bunch to toggle haunt modes.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T517, T214

Differential Revision: https://secure.phabricator.com/D2041