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/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!