REST API: Add locale suggestion endpoint & helpers for plugins and themes

[ryelle]

See WordPress/wporg-theme-directory#52 — I've moved rest-api-locale.php to mu-plugins/helpers/locale.php, and the endpoint to rest-api/endpoints/. In fact, I've updated the code to handle both theme and plugin requests, so that all this locale-banner-related code can stay in one place (this isn't exactly what was requested on the other PR, but once I started building it out as two similar endpoints, it made sense to share a base class & live here).

The endpoints:

• https://wordpress.org/themes/wp-json/wporg-themes/v1/locale-banner/ — Main theme directory banner
• https://wordpress.org/themes/wp-json/wporg-themes/v1/locale-banner/pulitzer — A specific theme request
• https://wordpress.org/plugins/wp-json/wporg-plugins/v1/locale-banner/ — Main plugin directory banner
• https://wordpress.org/plugins/wp-json/wporg-plugins/v1/locale-banner/recipe-block/ — A specific plugin request

These can also be used from Rosetta URLs

• https://eo.wordpress.org/themes/wp-json/wporg-themes/v1/locale-banner/
• https://eo.wordpress.org/plugins/wp-json/wporg-plugins/v1/locale-banner/recipe-block/

The routes should only appear on the relevant sites, so you can't request wordpress.org/themes/wp-json/wporg-plugins/v1/locale-banner/.

This leaves open the possibility to add a patterns variation later, if we wanted to.

To test

Same instructions as WordPress/wporg-theme-directory#52

Try out various combinations of header locale (Accept-Language header), theme, rosetta origin site, etc. This needs a sandbox to test for the locale database.

You can also try with the try/mu-plugins-locale-api branch on wporg-theme-directory, which should show the banner if your browser is set up to send alternate language suggestions.

There is also a debug parameter you can add to the endpoints to show more information about the request. For example,

$ curl 'https://es.wordpress.org/themes/wp-json/wporg-themes/v1/locale-banner/?debug=1' -H 'Accept-Language: en,ko-KR,es-ES,fr,eo,it,ru'

{
	"currentLocale": "es_ES",
	"suggestions": [
		"ko_KR",
		"es_ES",
		"fr_FR",
		"eo",
		"it_IT",
		"ru_RU"
	],
	"message": "The theme directory is also available in <a href=\"https:\/\/ko.wordpress.org\/themes\/\">한국어<\/a>, <a href=\"https:\/\/fr.wordpress.org\/themes\/\">Français<\/a>, <a href=\"https:\/\/eo.wordpress.org\/themes\/\">Esperanto<\/a>, <a href=\"https:\/\/it.wordpress.org\/themes\/\">Italiano<\/a>, 그리고 <a href=\"https:\/\/ru.wordpress.org\/themes\/\">Русский<\/a>."
}

[StevenDufresne]

I've taken a look at the code and left some minor suggestions. I don't know how much is moved code versus new so I'll let you determine what needs to be updated in regards to that. Otherwise, I feel comfortable merging this.

[ryelle]

I've made the requested updates and I think it's good to merge now, but I'll wait until tomorrow in case there's any more feedback or comment replies.