Label/title missing for interval and period field for accessibility

Problem/Motivation

The interval and period select elements are missing labels for accessibility.

Steps to reproduce

1. Turn on VO
2. Tab into the period and interval select

VO will read out the value but not what the value is for.

Proposed resolution

Similar to https://www.drupal.org/project/interval/issues/2352491 but for accessibility on 8.x, also covering the period field, appending the field title with the descriptive word of `count` or `period` for each field.

User interface changes

There should be not change for visual users.