Remove Firestore emulator [SCP-1968]

[eweitz]

This refactors tests to not use the Firestore emulator, following the team's determination that Firestore does not suit our needs. It basically undoes #26.

Like before emulator integration work, tests now intercept calls to methods that load to the database. This decreases coverage and confidence slightly, but makes the tests agnostic to database technology while still being quite robust to changes in Ingest Pipeline's internal implementation.

This satisfies SCP-1968.

[codecov]

Codecov Report

Merging #44 into master will increase coverage by 1.54%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #44      +/-   ##
=========================================
+ Coverage   65.15%   66.7%   +1.54%     
=========================================
  Files          11      11              
  Lines        1191     919     -272     
=========================================
- Hits          776     613     -163     
+ Misses        415     306     -109

Impacted Files	Coverage Δ
ingest/ingest_pipeline.py	43.41% <100%> (-3.52%)	⬇️
ingest/subsample.py	22.72% <0%> (-69.34%)	⬇️
ingest/gene_data_model.py	59.57% <0%> (-2.13%)	⬇️
ingest/cell_metadata.py	87.69% <0%> (-1.72%)	⬇️
ingest/validation/validate_metadata.py	83.05% <0%> (-1.42%)	⬇️
ingest/validation/validate_ontology_terms.py
ingest/__init__.py	0% <0%> (ø)
ingest/clusters.py	26.66% <0%> (+0.51%)	⬆️
ingest/ingest_files.py	87.37% <0%> (+0.65%)	⬆️

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[jlchang]

I confess, until I mock something myself I suspect I won't be able to skim through a PR on mocked responses quickly...
Knowing this is blocking Eno's progress, I'm not going to let my lack of expertise get in the way.

Other than some double quotes (which I believe Eric is already eliminating), I have no additional feedback. Hope to be a more useful reviewer in future.