Render user display names in trophy messages

Review Request #1255 — Created June 4, 2018 and updated — Latest diff uploaded

Review Board
Trophy message rendering has been reworked. Instead of a method that
every `TrophyType` subclass must implement, instead they must only add a
format string attribute. By default, the recipient's name is the only
provided parameter, but `TrophyType.format_display_text` can be
overridden to add extra context. All builtin trophies have been updated
to use this new behaviour.

Testing Done:
Ran unit tests.

Reviewed at

Did some stuff.