users: don't set inviter field; clear inviter field for most users.

* Don't set the inviter field for newly promoted users, or for Gold/Plat
  upgrades.

* Clear the inviter field for paid Gold/Plat upgrades, and for users who
  have a feedback or a modaction listing who invited them. This leaves
  about 600 remaining users with an inviter field with no other record
  of who invited them.

See #4750.

app/logical/user_promotion.rb

@@ -18,7 +18,6 @@ class UserPromotion
     user.level = new_level
     user.can_upload_free = can_upload_free unless can_upload_free.nil?
     user.can_approve_posts = can_approve_posts unless can_approve_posts.nil?
-    user.inviter = promoter
 
     create_user_feedback
     create_dmail

app/models/user_upgrade.rb

@@ -119,7 +119,7 @@ class UserUpgrade < ApplicationRecord
     end
 
     def upgrade_recipient!
-      recipient.update!(level: level, inviter: User.system)
+      recipient.update!(level: level)
     end
 
     def create_mod_action!