[Feat]: Fully configurable notifications control

[sashwathn]

Problem

Netdata Cloud UI only supports a set of combinations for enabling what notifications can be sent per room.

Description

I think we need to support on our UI an option to select exactly what notifications our users want to be sent.
@papazach @car12o @juacker @kapantzak : Here's a ticket that our paid customer has raised and I think it makes sense to provide this flexibility.

Please see screen shot, we are looking to see if a "bucket" can be added for "critical alerts, and unreachable". This would be nice to have due to how we plan to monitor our non-prod environment. We don'tont want notifications for all alerts, just the critical and unresponsive.

cc: @ralphm @shyamvalsan

Importance

nice to have

Value proposition

1. Provides full control and flexibility to our customers on what notifications should be received.

Proposed implementation

No response