DEV: Use method instead of constant for discobot badge names

This change refactors the code a bit so that a plugin could easily
replace which badge is awarded when completing the discobot new user
tutorial and advanced tutorial.

By adding a static method and putting the BADGE_NAME constant inside of
that method we can simply call that method now instead of the constant.
A plugin could then `class_eval` that method and replace it with
whatever badge name they choose. This is way cleaner than having the
plugin change the frozen constant! eeek.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/base.rb

@@ -74,7 +74,7 @@ module DiscourseNarrativeBot
               cancel_timeout_job(user)
 
               BadgeGranter.grant(
-                Badge.find_by(name: self.class::BADGE_NAME),
+                Badge.find_by(name: self.class.badge_name),
                 user
               )