rPa68b8daafde3

Avoid "Unknown Object (????)" for custom Select field values in Herald editor

Summary:
Custom Select field values are rendered as `Unknown Object (????)` when set as Herald rule conditions and editing the Herald rule.
This is unhelpful; at least display their key value.

Closes T15860

Test Plan: Create a `"type": "select"` custom field via `/config/edit/maniphest.custom-field-definitions/`, set up a Herald rule with a condition based on that custom field, edit the Herald rule.

Reviewers: O1 Blessed…