(re-revert) feat: add silencing notification

[mabdh]

No description provided.

[mabdh]

re-revert the PR #167

[coveralls]

Pull Request Test Coverage Report for Build 4080725895

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A quick fix for this PR: rebase it; your next report should be accurate.

• 614 of 790 (77.72%) changed or added relevant lines in 26 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.003%) to 77.209%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/notification/notification.go	21	22	95.45%
internal/api/v1beta1/alert.go	12	13	92.31%
internal/store/postgres/subscription.go	3	6	50.0%
core/notification/dispatch_subscriber_service.go	107	111	96.4%
core/silence/silence.go	37	41	90.24%
internal/api/v1beta1/silence.go	62	66	93.94%
internal/store/postgres/alerts.go	9	13	69.23%
core/notification/service.go	46	51	90.2%
plugins/providers/cortex/service.go	0	5	0.0%
core/silence/type.go	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/notification/notification.go	1	91.67%
plugins/queues/postgresq/queue.go	2	72.62%
core/notification/service.go	3	82.29%

Totals
Change from base Build 4072599184:	0.003%
Covered Lines:	3381
Relevant Lines:	4379

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💛 - Coveralls