Fixing meta boxes added too late or too early

[dparker1005]

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Changes proposed in this Pull Request:

We have seen plugin conflicts where either our meta boxes aren't being loaded on "edit post" pages with some page builders including Avada, or where add_meta_box() is being called before the function is defined. The latter can cause a fatal error when updating to 3.0.

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Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[ideadude]

Looks good. Be sure to rebuild blocks and test extensively on the block editor, class editor, and some CPT pages (block and classic editor versions).