Make ghost event instances render better on the event list

Summary:
Ref T11326. Currently, we render "E (99)" for ghost instances, which is meaningless and inconsistent.

Render these more sensibly and consistently.

Test Plan: Viewed event list, saw reasonable monograms / object names.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16296