Add support for w.org translations

[brashrebel]

See details here. If we ditch load_plugin_textdomain(), add the domain and path to the header, change the domain to match the slug and require WP 4.6 or higher we can let community translations on w.org work instead of shipping them ourselves.

[JJJ]

I wonder if we can update the slug on WordPress.org to sugar-calendar instead, or just create a new repo over there.

[pippinsplugins]

Changing slugs on WordPress.org is not permitted.

[JJJ]

I looked into this a bit more today. All of the above options & work are still the same.

• If we changed the text domain, I'd want to script it as a Grunt task so that Standard & Lite could continue to be cross-merged without needing to update these manually
• If we created a new repo, we'd need a way to migrate people to the new one, but we wouldn't need to complicate the build process
• I'm unsure if, perhaps, what we have actually is enough for now, because we do have a GlotPress repo, and there are pending translations (here's Dutch for example)

I don't really want to do either of the two top things, but translations are extremely helpful and important for everyone. We should do something eventually.