feat(core): pass locale to custom preview templates

[delucis]

Closes #5911

Summary

Pass the currently selected locale to the editor preview pane to allow custom preview templates to adapt to different locales (see #5911).

Test plan

I created a collection in dev-test/config.yml with i18n enabled.

In dev-test/index.html I registered a custom preview template to display the passed locale prop:

CMS.registerPreviewTemplate(
  'posts',
  createClass({
    render() {
      return h('p', {}, this.props.locale);
    },
  }),
);

The locale string displayed in the preview pane and toggling between the locale currently being edited updated the preview string.

I’d love to add an integration test for a custom preview template and check the props it receives, but I couldn’t see any existing tests for this.

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.
• Code is formatted via running yarn format.
• Tests are passing via running yarn test.
• The status checks are successful (continuous integration). Those can be seen below.

A picture of a cute animal (not mandatory but encouraged)

A locale locust:

By Charles J. Sharp - Own work, from Sharp Photography, sharpphotography, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=59068817

[delucis]

Netlify Deploy Preview seems to be failing on something unrelated:

lerna ERR! yarn run build exited 1 in 'netlify-cms-backend-bitbucket'

Is this something random or could it be related to my one-line change in front-end code?

[erezrokah]

Is this something random or could it be related to my one-line change in front-end code?

Thanks @delucis #5914 should fix it

[erezrokah]

Thanks @delucis, very useful 🎉