State: Deduplicate notices by distinct ID

[aduth]

This pull request seeks to update the behavior of notices reducer to respect ID as a unique identifier, removing duplicate entries. The revised behavior appends the latest notice as the last value in the set, removing any prior entries which share the same ID.

Testing instructions:

Ensure that unit tests pass:

npm run test

Currently there do not appear to be any situations in which a duplicate notice would be shown, only because the behavior of saving explicitly calls to remove the existing notice.

gutenberg/editor/effects.js

Line 76 in c4444e4

dispatch( removeNotice( SAVE_POST_NOTICE_ID ) );

However, this has shown to be an issue in #3378 where saving a reusable block displays a notice with a common ID.

[codecov]

Codecov Report

Merging #3764 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3764   +/-   ##
=======================================
  Coverage   37.67%   37.67%           
=======================================
  Files         279      279           
  Lines        6737     6737           
  Branches     1226     1226           
=======================================
  Hits         2538     2538           
  Misses       3538     3538           
  Partials      661      661

Impacted Files	Coverage Δ
editor/reducer.js	90.37% <100%> (ø)	⬆️

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