Change assistive technology focus after Show more / less

Problem/Motivation

After clicking the "Show more" or "Show less" link on a Facet that has a limit, keyboard / screenreader focus remains on the "Show more" or "Show less" link; so the next thing that the screenreader reads is the next facet block.

Proposed resolution

Set focus on the first checkbox in the facet after expanding or collapsing the facet.

Remaining tasks

1. Write a patch
2. Review and feedback
3. RTBC and feedback
4. Commit

User interface changes

None.

API changes

None.

Data model changes

None.