More robust PuSH subscription refreshes (#2799)

* Fix #2473 - Use sidekiq scheduler to refresh PuSH subscriptions instead of cron

Fix an issue where / in domain would raise exception in TagManager#normalize_domain

PuSH subscriptions refresh done in a round-robin way to avoid hammering a single
server's hub in sequence. Correct handling of failures/retries through Sidekiq (see
also #2613). Optimize Account#with_followers scope. Also, since subscriptions
are now delegated to Sidekiq jobs, an uncaught exception will not stop the entire
refreshing operation halfway through

Fix #2702 - Correct user agent header on outgoing http requests

* Add test for SubscribeService

* Extract #expiring_accounts into method

* Make mastodon:push:refresh no-op

* Queues are now defined in sidekiq.yml

* Queues are now in sidekiq.yml

spec/services/follow_service_spec.rb

@@ -53,10 +53,11 @@ RSpec.describe FollowService do
     end
 
     describe 'unlocked account' do
-      let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob', domain: 'example.com', salmon_url: 'http://salmon.example.com')).account }
+      let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob', domain: 'example.com', salmon_url: 'http://salmon.example.com', hub_url: 'http://hub.example.com')).account }
 
       before do
         stub_request(:post, "http://salmon.example.com/").to_return(:status => 200, :body => "", :headers => {})
+        stub_request(:post, "http://hub.example.com/").to_return(status: 202)
         subject.call(sender, bob.acct)
       end
 
@@ -70,6 +71,10 @@ RSpec.describe FollowService do
           xml.match(TagManager::VERBS[:follow])
         }).to have_been_made.once
       end
+
+      it 'subscribes to PuSH' do
+        expect(a_request(:post, "http://hub.example.com/")).to have_been_made.once
+      end
     end
   end
 end