Allow application to be eager loaded without ActionCable

[adamsosterics]

Issue #, if available: #200

Summary

The files in app/channels/activity_notification triggered Zeitwerk errors when the app didn't use ActionCable.
This PR changes these files, so the classes are always defined, no matter if ActioneCable is defined or not.

Other Information

[adamsosterics]

I'm not 100% sure if this is the best solution but these modifications allowed me to load the gem.

[coveralls]

coverage: 99.732% (-0.03%) from 99.759%
when pulling bd62785 on adamsosterics:action_cable_eager_load
into 8443d2b on simukappu:master.

[adamsosterics]

I tried to write a test which eager loads the gem with and without ActionCable included, but it didn't really work out.
I added this test:

it 'eager loads all files' do
   expect{ Zeitwerk::Loader.eager_load_all }.not_to raise_error
end

But then I needed a way to initialize the app with and without ActionCable, but as you load the app in the spec_helper, I'm not sure hot to do it properly.
Should I try to test it? Do you have a recommendation for that?
Or the coverage drop is acceptable?
Or are you looking at a totally different solution?