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/models/allowed_pm_user.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class AllowedPmUser < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :allowed_pm_user, class_name: "User"
+end
+
+# == Schema Information
+#
+# Table name: allowed_pm_users
+#
+#  id                 :bigint           not null, primary key
+#  user_id            :integer          not null
+#  allowed_pm_user_id :integer          not null
+#  created_at         :datetime         not null
+#  updated_at         :datetime         not null
+#
+# Indexes
+#
+#  index_allowed_pm_users_on_allowed_pm_user_id_and_user_id  (allowed_pm_user_id,user_id) UNIQUE
+#  index_allowed_pm_users_on_user_id_and_allowed_pm_user_id  (user_id,allowed_pm_user_id) UNIQUE
+#