Enqueue the datepicker UI when using a date field in the front-end

[jom]

Changes proposed in this Pull Request:

• Enqueue the jQuery UI CSS when using the datepicker on the front-end.

Testing instructions:

• Add a datepicker field and verify it is styled correctly on the front-end.

add_filter( 'submit_job_form_fields', function( $fields ) {
	$fields['job']['fancy_date'] = array(
		'label'       => __( 'Fanciest of Dates', 'wp-job-manager' ),
		'type'        => 'date',
		'required'    => false,
		'priority'    => 5
	);
	return $fields;
} );

Builds off of #1353

[alexsanford]

Looks great! I guess Application Deadline must do this itself? There doesn't seem to be any style issues with that date field (thus why I missed this issue). Maybe we should consider removing that styling from Application Deadline in favour of this solution?

(Also I edited the description to link to the correct PR 😄 )