Drop forum subscriptions.

Few people used forum subscriptions (only around 100), and even fewer
people were subscribed to active threads. Most subscriptions were for
old threads that will never be bumped again. The implementation also had
a few problems:

* Unsubscribe links in emails didn't work (they unset the user's
  receive_email_notifications flag, but forum subscriptions didn't
  respect this flag).
* Some users had invalid email addresses, which caused notifications to
  bounce. There was no mechanism for preventing bounces.
* The implementation wasn't scalable. It involved a daily linear scan
  over _all_ forum subscriptions looking for any topics that had been updated.

test/factories/forum_subscription.rb

@@ -1,3 +0,0 @@
-FactoryBot.define do
-  factory(:forum_subscription)
-end

test/mailers/previews/user_mailer_preview.rb

@@ -3,12 +3,4 @@ class UserMailerPreview < ActionMailer::Preview
     dmail = User.admins.first.dmails.first
     UserMailer.dmail_notice(dmail)
   end
-
-  def forum_notice
-    topic = ForumTopic.first
-    posts = topic.posts
-    user = topic.creator
-
-    UserMailer.forum_notice(user, topic, posts)
-  end
 end