Improved table filtering

[illusional]

• Move filtering classes to own namespace
• Add more nested fields to sample / sequencing group filtering

NB: This is built on #742, so there will be some strange diffs, because some of the participant query changes were created there.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.04762% with 47 lines in your changes missing coverage. Please review.

Project coverage is 79.33%. Comparing base (34280ee) to head (0506e90).

Files	Patch %	Lines
db/python/db_filters/generic.py	86.15%	27 Missing :warning:
db/python/tables/sample.py	82.45%	10 Missing :warning:
db/python/tables/sequencing_group.py	87.50%	5 Missing :warning:
db/python/layers/seqr.py	60.00%	2 Missing :warning:
db/python/layers/web.py	85.71%	2 Missing :warning:
db/python/layers/sequencing_group.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## ourdna/update-table-view-and-filtering #823 +/- ## ========================================================================== + Coverage 79.20% 79.33% +0.12% ========================================================================== Files 162 168 +6 Lines 14040 14168 +128 ========================================================================== + Hits 11121 11240 +119 - Misses 2919 2928 +9 ```

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