Changing term Invoice to Order for all places that will not break backwards compatibility.

[kimcoleman]

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Changes proposed in this Pull Request:

We know that using the term Invoice isn't really the correct work for what are essentially Orders. This is the first step in a multi-phase project to eliminate the incorrect term Invoice from our code.

This work does not include the Emails. Most of those changes will break some compatibility and require upgrade scripts as well as deprecations and fallbacks. That work will be done in phase 2.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

• ENHANCEMENT: Adjusting various places where the word Invoice was used to now use the correct term: Order.