[lib] Simplify messageTitle and introduce default for robotext

Summary: This diff switches messageTitle to returning EntityText as preparation for ENS resolution. I take this opportunity to simplify the logic for global_viewer, and to delete most of the implementations of messageTitle in favor of a default for robotext.

Test Plan: Tested in combination with following diffs, where the message title is actually used. Also Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6560