Adjust the WC_Admin_Notices to support multisite setups

[Konamiman]

Changes proposed in this Pull Request:

The WC_Admin_Notices uses a static array to cache the names of the existing notices locally (the $notices field). In multisite setups this will cause the sequence of calls WC_Admin_Notices::add_custom_notice() - switch_to_blog() - WC_Admin_Notices::store_notices() to generate a woocommerce_admin_notices option in the second site with the values of the option names for the first site.

While this doesn't cause notices to actually appear in wrong sites (each notice gets an additional woocommerce_admin_notice_<name> that is created in the proper site and is needed for the notice to actually be displayed), it causes the woocommerce_admin_notices option to get unnecessary values, which can lead to confussion (and in extreme cases degrade the site performance).

This pull request fixes this by transforming the $notices field into an associative array where keys are site ids and values are arrays of notice names (for multisite only: for single site setups the original behavior is kept, so $notices is still a plain names array).

How to test the changes in this Pull Request:

Create a site in multisite mode, add one additional site, run wp shell in the command line and execute this:

WC_Admin_Notices::add_custom_notice('site_1', 'Notice in site 1');
WC_Admin_Notices::store_notices();
switch_to_blog(2);
WC_Admin_Notices::add_custom_notice('site_2', 'Notice in site 2');
WC_Admin_Notices::store_notices();
var_dump(get_option('woocommerce_admin_notices'));

Without this fix you'll get an array with two entries, site_1 and site_2. With the fix you'll get an array with only site_2.

You can also browse the admin area of both sites and verify that each site is still displaying its own notice only.

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Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

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[github-actions]

Hi @lsinger,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[github-actions]

Test Results Summary

Commit SHA: 202375b

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 39s
E2E Tests	341	0	0	10	0	351	7m 36s

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To view the full API test report, click here.
To view the full E2E test report, click here.
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[lsinger]

Looks good and tests as intended. Also tested with a non-multisite blog to make sure that's not affected. Merge when CI has passed.