Compat Checker – Adds perma-dismissible warning notices

[ibndawood]

Changes proposed in this Pull Request:

Closes #84 .

In this PR:

• We use WC_Admin_Notices to add dismissible warnings stored in the usermeta table.
• If WC_Admin_Notices is unavailable due to WooCommerce being de-activated and the notice is not a warning, we display the non-dismissible error notice.
• Fixes a couple of phpcs warnings.

Screenshots:

Detailed test instructions:

1. Setup Compat Checker in a WooCommerce extension by following the Getting Started section.
2. Set the WC Tested upto of the WooCommerce extension plugin header file to less than the current active WooCommerce version.
3. Activate the extension and notice the warning as shown in the above screenshot.
4. Click on the Dismiss button to dismiss the warning.
5. Visiting the page again should not display this warning.

Additional details:

• The PR uses add_custom_notice method of WC_Admin_Notices.
• WC_Admin_Notices appear only in the plugins, dashboard and WooCommerce pages.

Changelog entry

Add - Perma-dismissible warning notices.

[layoutd]

👍🏻 This looks OK!

• The message was displayed only once (and the dismissal was stored in the usermeta.
• From what I can tell, the accent color is just based on the store's main theme, right?
• I had to manually copy the new files into the extension vendor/woocommerce/grow directory to test.

[ibndawood]

Thank you, @layoutd, for your review. Yes, the color is not set by our code but the custom notices are set by WooCommerce itself.