Revive PR: Adds mechanism to prevent spam from unhealthy notifications

[cieciurm]

What this PR does / why we need it:

This PR revives #592
Original description:

Adds mechanism to prevent spam from unhealthy notifications.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

• Merged with master
• Tests fixed
• Formatting fixed
• Public API approved

Does this PR introduce a user-facing change?:

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[cieciurm]

The CI seems to be flaky - should_return_too_many_requests_status_using_default_server_max_active_requests suddenly failed, any chance you could re-run the CI? cc @sungam3r

[cieciurm]

PR adjusted
cc @sungam3r

EDIT: The flaky test should_return_too_many_requests_status_using_default_server_max_active_requests failed yet again. Could you re-run the workflow?

[codecov-commenter]

Codecov Report

Merging #1877 (7a0ff36) into master (a65b73c) will decrease coverage by 0.19%.
The diff coverage is 20.00%.

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@@            Coverage Diff             @@
##           master    #1877      +/-   ##
==========================================
- Coverage   66.94%   66.76%   -0.19%     
==========================================
  Files         228      228              
  Lines        8135     8163      +28     
  Branches      573      577       +4     
==========================================
+ Hits         5446     5450       +4     
- Misses       2545     2567      +22     
- Partials      144      146       +2     

Flag	Coverage Δ
UI	65.73% <20.00%> (-0.35%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/HealthChecks.UI/Configuration/Settings.cs	82.82% <20.00%> (-3.35%)	⬇️
...I/Core/HostedService/HealthCheckReportCollector.cs	60.56% <20.00%> (-5.23%)	⬇️

... and 1 file with indirect coverage changes

[cieciurm]

@sungam3r - you suggestions are applied now :)

[cieciurm]

Public API has been sorted alphabetically and workflow succeeded 🎉