Bugfixes for Exchange Objectives

-Will no longer be assigned to latejoiners
-Objective description will report the other agent's real_name
-Objective description will report what job the other agent is
-Objective description will properly rename itself if the other agent is renamed (e.g. clowns, mimes)
-Explanation text made more noticeable
-Cleaned up the code a bit

Fixes #3927
Fixes #3786