fixes #1278

[blsouthcott]

Proposed Changes

• Added a test for the copy nutrition plan function/endpoint
• Refactored copy nutrition plan function to avoid potential race condition that occurs, causing meals and meal items not to be copied when creating a copy of the nutrition plan

Notes:
The way the function is currently written in the master branch, the newly added test does not pass. The test does pass with the changes in this branch.

I was able to reproduce the issue locally, but only inconsistently. I was able to reliably reproduce it by creating a new account, adding a plan, and trying to copy that plan. However, I wasn’t able to consistently reproduce the issue for established accounts.

When I stepped through the code with a debugger, I didn’t encounter the issue. So I think a race condition can occur where plan_copy and meal_copy are still referencing the original plan and meal objects, even though those objects should be updated with the primary key for the new database entry. I suspect that creating new objects using the values of the fields of the original objects more reliably avoids a potential race condition because there are two completely separate objects, whereas currently plan and plan_copy, and meal and meal_copy, each reference the same object.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR
  (e.g. database migration)? No