Overwrite order URL for admins in order details emails

[emdashcodes]

See Automattic/wp-calypso#16660.

The emails being sent to the store admin (which is also our only notification right now) links to wp-admin instead of Calypso. This PR adds a hotfix for overwriting the templates with our own, linking to WP.com.

To Test:

• Upload/install this branch over your existing wc-api-dev plugin.
• Go through the check-out process and make sure you receive an admin order notification email.
• Make sure that the link on the Order # goes to WP.com (make sure you are looking at the admin notification, and not the notification that is sent to the customer).
• To test the plaintext email (which is not currently an option we offer on .com, but I provided here anyway), you can go to wp-admin > WooCommerce > Settings > Emails > New Order and then set it to plaintext.
• Repeat the above steps.

[ryelle]

One comment about the URL builder, but otherwise it seemed to test OK.

My api-dev test site is just pressable, not AT, so I got redirected back to /stats/ when I clicked the email links, but if I switched them to wpcalypso they're fine.

[ryelle]

Probably not something to worry about immediately, but this won't work for URLs where WP is in a subfolder. You can steal Jetpack's build_raw_urls function though (and it looks like this is usually passed get_home_url, not get_site_url).

[emdashcodes]

Good catch and suggestion. I think it's a good idea to handle this now so we don't run into issues later.

[emdashcodes]

I'll re-open this after the plugin split.

[emdashcodes]

@ryelle I made the change that should allow for sites w/ paths later on. Can you take a look/test and ✅ if everything looks good?

[ryelle]

Code looks good 👍 I don't actually have a subfolder site to test on, but I know that pattern works for jetpack, so I think you're good to merge