CRM: 3404 - address syntax differences between SQLite and MySQL

[tbradsha]

Resolves Automattic/zero-bs-crm#3404 - address syntax differences between SQLite and MySQL

Proposed changes:

Transaction queries were outright failing when using SQLite. The underlying causes:

1. We use transaction as a table alias in some of our queries. This is a reserved keyword in both MySQL and SQLite, but apparently SQLite is more strict. This PR changes the table alias to transactions.
2. MySQL and SQLite have slightly different syntax for GROUP_CONCAT(). I logged the issue here, and in the meantime created build_group_concat() to build the proper string depending on the database engine.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

The most painless way to do this is to set up SQLite via the SQLite Database Integration plugin. Install that on a WordPress site, initialise the site, and then you should be good to go.

In trunk, the dashboard shows a null count, the transaction list shows no transactions even if you create one, and creating/viewing a transaction shows an error (see issue for screenshots).

In the fix/crm/3404-sqlite_transactions branch, the dashboard shows the correct count, the transaction list shows transactions, and one can create/view transactions.

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[coder-karen]

Tests well for me.