FEATURE: Allow List for PMs (#10270)

* FEATURE: Allow List for PMs

This feature adds a new user setting that is disabled by default that
allows them to specify a list of users that are allowed to send them
private messages. This way they don't have to maintain a large list of
users they don't want to here from and instead just list the people they
know they do want. Staff will still always be able to send messages to
the user.

* Update PR based on feedback

app/services/user_updater.rb

@@ -37,6 +37,7 @@ class UserUpdater
     :include_tl0_in_digests,
     :theme_ids,
     :allow_private_messages,
+    :enable_allowed_pm_users,
     :homepage_id,
     :hide_profile_and_presence,
     :text_size,
@@ -164,6 +165,10 @@ class UserUpdater
         update_ignored_users(attributes[:ignored_usernames])
       end
 
+      if attributes.key?(:allowed_pm_usernames)
+        update_allowed_pm_users(attributes[:allowed_pm_usernames])
+      end
+
       name_changed = user.name_changed?
       if (saved = (!save_options || user.user_option.save) && user_profile.save && user.save) &&
          (name_changed && old_user_name.casecmp(attributes.fetch(:name)) != 0)
@@ -225,6 +230,28 @@ class UserUpdater
     end
   end
 
+  def update_allowed_pm_users(usernames)
+    #return unless guardian.can_ignore_users?
+
+    usernames ||= ""
+    desired_usernames = usernames.split(",").reject { |username| user.username == username }
+    desired_ids = User.where(username: desired_usernames).where(admin: false, moderator: false).pluck(:id)
+    if desired_ids.empty?
+      AllowedPmUser.where(user_id: user.id).destroy_all
+    else
+      AllowedPmUser.where('user_id = ? AND allowed_pm_user_id not in (?)', user.id, desired_ids).destroy_all
+
+      # SQL is easier here than figuring out how to do the same in AR
+      DB.exec(<<~SQL, now: Time.zone.now, user_id: user.id, desired_ids: desired_ids)
+        INSERT into allowed_pm_users(user_id, allowed_pm_user_id, created_at, updated_at)
+        SELECT :user_id, id, :now, :now
+        FROM users
+        WHERE id in (:desired_ids)
+        ON CONFLICT DO NOTHING
+      SQL
+    end
+  end
+
   private
 
   attr_reader :user, :guardian