Add input tree and expandable facets

[anselmbradford]

Input tree is used in
https://www.consumerfinance.gov/consumer-tools/educator-tools/youth-financial-education/teach/activities
https://www.consumerfinance.gov/data-research/consumer-complaints/search/

Expandable facet is used in
https://www.consumerfinance.gov/consumer-tools/educator-tools/youth-financial-education/teach/activities
https://www.consumerfinance.gov/data-research/consumer-complaints/search/
https://www.consumerfinance.gov/data-research/prepaid-accounts/search-agreements/

Additions

• Add input tree component.
• Add expandable facet component.

Testing

1. PR preview should have an input tree example in the Fieldsets (here).
2. PR preview should have an expandable facet example that contains an input tree (here).

Screenshots

[netlify]

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[anselmbradford]

This is a draft of the longer-term fix for cfpb/consumerfinance.gov#7659

Additional work that needs to happen:

• The expandable facet has nested checkboxes. There should also be an expandable facet with only single checkboxes that do not have children.
• There should be an option is_collapsed_for_mobile, which collapses an otherwise expanded expandable facet at mobile (see https://github.com/cfpb/consumerfinance.gov/blob/7b6190d4c43c8fcffb5a126e1285ecb1e731344d/cfgov/unprocessed/apps/teachers-digital-platform/js/expandable-mobile.js)