Aggregate and display unknown values

[macfarlandian]

Description of the change

When demographic categories are reported as unknown (which can happen due to data quality or other issues), counts the number of occurrences and displays them in a footnote.

(The snapshot change occurred because one of the fixtures randomly had a few unknowns in it. I took them out to more consistently isolate the cases where unknowns are present or absent.)

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Configuration change (adjusts configuration to achieve some end related to functionality, development, performance, or security)

Related issues

Closes #330

Checklists

Development

These boxes should be checked by the submitter prior to merging:

• Manual testing against realistic data has been performed locally

Code review

These boxes should be checked by reviewers prior to merging:

• This pull request has a descriptive title and information useful to a reviewer
• This pull request has been moved out of a Draft state, has no "Work In Progress" label, and has assigned reviewers
• Potential security implications or infrastructural changes have been considered, if relevant

[coveralls]

Pull Request Test Coverage Report for Build 681350396

• 70 of 70 (100.0%) changed or added relevant lines in 14 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 73.36%

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Totals
Change from base Build 680511106:	0.5%
Covered Lines:	2759
Relevant Lines:	3639

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💛 - Coveralls