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/autoload/jobs/onceoff/grant_badges.rb

@@ -5,11 +5,11 @@ module Jobs
     class GrantBadges < ::Jobs::Onceoff
       def execute_onceoff(args)
         new_user_track_badge = Badge.find_by(
-          name: ::DiscourseNarrativeBot::NewUserNarrative::BADGE_NAME
+          name: ::DiscourseNarrativeBot::NewUserNarrative.badge_name
         )
 
         advanced_user_track_badge = Badge.find_by(
-          name: ::DiscourseNarrativeBot::AdvancedUserNarrative::BADGE_NAME
+          name: ::DiscourseNarrativeBot::AdvancedUserNarrative.badge_name
         )
 
         PluginStoreRow.where(