End-to-end test of notification service & updating the subscription flow

[mabdh]

Is your feature request related to a problem? Please describe.
Notification service has been implemented but the subscription and alert flow was still not well adjusted. This means, all subscriptions will be synced to the provider (cortex) although that subscription is not an alert subscription.

Describe the solution you'd like

• Deprecate subscription synchronization from cortex and only keep alert history webhook route in provider (alertmanager cortex).
• Add end-to-end test for notification service, from subscription to notification handling.