Fix: duplicate meta keys

[alaca]

Resolves GIVE-137

Description

This PR resolves the issue of duplicate donation forms. The problem was in the logic of the get_earnings method combined with the donation transfer logic which resulted in the meta key being duplicated on every page load. That also affected the donation forms list table which was displaying duplicated results.

The issue is resolved by using update_post_meta instead of add_post_meta function, and by adding a migration for deleting duplicated meta keys (_give_form_earnings, _give_form_sales) and setting the correct values for _give_form_earnings is added.

Affects

Donation Forms list table
Donation Form data transfer

Testing Instructions

The best way to test this is on an existing site that has this problem.

Create a new site by using Local or something similar.
Install All-In-On WP Migration plugin.
Download the site backup and import it (https://drive.google.com/file/d/16SIlaCQ-1nMvneFpYvdLStA2_ovtQ6vG/view?usp=sharing).
After import, verify that the problem exists by checking the donation forms list table, or even better by querying the give_formmeta table. Something like:

select form_id, meta_key from wp_give_formmeta where form_id = FORM_ID and ( meta_key = '_give_form_sales' or  meta_key = '_give_form_earnings');

Now delete or disable the existing Give plugin and install Give from GitHub repo.
Switch to this branch fix/duplicate-meta-keys
Check the donation forms list table.

Pre-review Checklist

• Relevant @unreleased tags included in DocBlocks
• Self Review of code and UX completed

[jonwaldstein]

@alaca should we also update TransferDonations.php - so that we're falling back on 0 revenue when empty?

[alaca]

@jonwaldstein good catch. Although it's not necessary, we should probably update it. Resolved!

[jonwaldstein]

@alaca great job! ready for QA 🌈

[rickalday]

Passed manual QA test