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/advanced_user_narrative.rb

@@ -98,6 +98,10 @@ module DiscourseNarrativeBot
       }
     }
 
+    def self.badge_name
+      BADGE_NAME
+    end
+
     def self.reset_trigger
       I18n.t('discourse_narrative_bot.advanced_user_narrative.reset_trigger')
     end

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
               )
 

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

@@ -117,6 +117,10 @@ module DiscourseNarrativeBot
       }
     }
 
+    def self.badge_name
+      BADGE_NAME
+    end
+
     def self.search_answer
       ':herb:'
     end