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/subscribe_service_spec.rb

@@ -0,0 +1,38 @@
+require 'rails_helper'
+
+RSpec.describe SubscribeService do
+  let(:account) { Fabricate(:account, username: 'bob', domain: 'example.com', hub_url: 'http://hub.example.com') }
+  subject { SubscribeService.new }
+
+  it 'sends subscription request to PuSH hub' do
+    stub_request(:post, 'http://hub.example.com/').to_return(status: 202)
+    subject.call(account)
+    expect(a_request(:post, 'http://hub.example.com/')).to have_been_made.once
+  end
+
+  it 'generates and keeps PuSH secret on successful call' do
+    stub_request(:post, 'http://hub.example.com/').to_return(status: 202)
+    subject.call(account)
+    expect(account.secret).to_not be_blank
+  end
+
+  it 'fails silently if PuSH hub forbids subscription' do
+    stub_request(:post, 'http://hub.example.com/').to_return(status: 403)
+    subject.call(account)
+  end
+
+  it 'fails silently if PuSH hub is not found' do
+    stub_request(:post, 'http://hub.example.com/').to_return(status: 404)
+    subject.call(account)
+  end
+
+  it 'fails loudly if there is a network error' do
+    stub_request(:post, 'http://hub.example.com/').to_raise(HTTP::Error)
+    expect { subject.call(account) }.to raise_error HTTP::Error
+  end
+
+  it 'fails loudly if PuSH hub is unavailable' do
+    stub_request(:post, 'http://hub.example.com/').to_return(status: 503)
+    expect { subject.call(account) }.to raise_error
+  end
+end