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 ⚠️
db/python/tables/sample.py	82.45%	10 Missing ⚠️
db/python/tables/sequencing_group.py	87.50%	5 Missing ⚠️
db/python/layers/seqr.py	60.00%	2 Missing ⚠️
db/python/layers/web.py	85.71%	2 Missing ⚠️
db/python/layers/sequencing_group.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@                            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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[milo-hyben]

From the quick review and as per our quick chat, I would only rename 'db_filters' to 'filters'
This reads better:
from db.python.filters import GenericFilter
than:
from db.python.db_filters import GenericFilter

Will wait till you merge both PRs into one.