Rename or replace gutenberg_get_jed_locale_data() usage

[swissspidy]

gutenberg_get_jed_locale_data() is now wp_get_jed_locale_data() in WordPress core.

The plugin currently fatals because of that.

The function should be renamed in the plugin and perhaps even considered to replaced in WP 5.0+ as per https://make.wordpress.org/core/2018/11/09/new-javascript-i18n-support-in-wordpress/.

See also #1604.

[swissspidy]

Either way, it doesn't hurt to add some function_exists() checks as is already done in \AMP_Post_Meta_Box.

[westonruter]

And the function has now been removed from core. We need to address this again.

See https://core.trac.wordpress.org/changeset/43910

[westonruter]

@swissspidy Can you figure out what needs to be done here for the 1.0-RC3 release tomorrow? /cc @kienstra