Misc spec coverage improvements (#2821)

* Dont use raise_error by itself (avoids warning)

* Add coverage for AccountFilter

* Improve coverage and refactor for Subscription#lease_seconds

* Improve coverage and refactor for NotificationMailer

* Simplify assignment of min/max threshold on subscription

spec/services/subscribe_service_spec.rb

@@ -33,6 +33,6 @@ RSpec.describe SubscribeService do
 
   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
+    expect { subject.call(account) }.to raise_error(/Subscription attempt failed/)
   end
 end