Refactor families + add family participants to graphql

[illusional]

I got carried away, but broadly:

• Refactor the family layer + table to support the newer flavour of querying (+ cleanup a few methods)
• Add family participants to the graphql schema
• Remove the type: ignore for the graphql schema, so there's better type checking there.

Will self-review, then assign out soon.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.36721% with 98 lines in your changes are missing coverage. Please review.

Project coverage is 77.15%. Comparing base (8ee989f) to head (83eb562). Report is 2 commits behind head on dev.

Files	Patch %	Lines
models/models/family.py	64.44%	48 Missing :warning:
db/python/layers/family.py	66.03%	18 Missing :warning:
api/graphql/schema.py	83.33%	10 Missing :warning:
db/python/tables/family_participant.py	77.14%	8 Missing :warning:
db/python/tables/family.py	74.07%	7 Missing :warning:
db/python/utils.py	82.60%	4 Missing :warning:
db/python/layers/participant.py	87.50%	2 Missing :warning:
api/routes/family.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #740 +/- ## ========================================== + Coverage 76.47% 77.15% +0.68% ========================================== Files 148 157 +9 Lines 11919 12945 +1026 ========================================== + Hits 9115 9988 +873 - Misses 2804 2957 +153 ```

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[illusional]

Thanks @EddieLF! Yeah, a bit of extra refactoring here to bring it in line with other classes. Yep, all your points are exactly correct! I've made a few changes per your suggestions, and to improve code coverage on some of the things I've changed (resulting in more changes).

I'm going to merge this one after #615, because I'll need to update a few things that I've changed here - so if you want to re-review, feel free to do it after that. Let me know if you're not comfy with approving this (though I think you're in a great position too :) ).

Thanks for the review!