Adding unified streamable notifications

app/channels/application_cable/channel.rb

app/controllers/api/v1/notifications_controller.rb

app/lib/feed_manager.rb

app/mailers/notification_mailer.rb

app/models/notification.rb

app/services/favourite_service.rb

app/services/follow_service.rb

app/services/notify_service.rb

app/services/process_feed_service.rb

app/services/process_interaction_service.rb

app/services/process_mentions_service.rb

app/services/reblog_service.rb

app/views/api/v1/notifications/index.rabl

app/views/api/v1/notifications/show.rabl

config/routes.rb

db/migrate/20161119211120_create_notifications.rb

db/schema.rb

spec/fabricators/notification_fabricator.rb

spec/mailers/notification_mailer_spec.rb

spec/models/notification_spec.rb