Live previews don't work for oEmbed fields within a repeatable group #1157

Expected Behavior:

When an oembed field type is present within a repeatable group, it should function as normal.

Actual Behavior:

The live Ajax-powered embed preview does not function, because the call to cmb2_ajax() never triggers, and thus its Ajax handlers are never registered. Ref:

Steps to reproduce (I have confirmed I can reproduce this issue on the trunk branch):

1. Register a repeatable group and place an oembed field type in it.
2. Paste an embed URL into the oEmbed URL input field.
3. Observe that the live preview doesn't function. The Ajax call responds with 0 because there's no registered Ajax handler for the oEmbed preview.

CMB2 Field Registration Code:

add_action( 'cmb2_init', function() {
	// Meta box
	$cmb_main_box = new_cmb2_box( [
		'id'           => 'main_box',
		'title'        => 'Main Content',
		'object_types' => [
			'post',
		],
	] );

	// Repeatable embed group
	$cmb_embed_group = $cmb_main_box->add_field( [
		'id'         => 'embeds',
		'repeatable' => true,
		'type'       => 'group',
	] );

	// Repeatable field: embed
	$cmb_main_box->add_group_field( $cmb_embed_group, [
		'id'      => 'url',
		'name'    => 'Embed URL',
		'type'    => 'oembed',
	] );
} );