273 use a single dbt venv for all clients

[fatchat]

deployment plan:

1. create /home/ddp/dbt/venv/ and install dbt-core==1.4.5, dbt-postgres==1.4.5, dbt-bigquery==1.4.3
2. set the DBT_VENV variable in the .env for django
3. release the code change from this PR
4. update Prefect's dbt-core-operation blocks' commands to point to the new dbt binary at /home/ddp/dbt/venv/bin/dbt
5. delete the various /home/ddp/clientdbts/*/venv

until we do (5) we have time to do (4) - new orgs signed up will use the central dbt and existing orgs will use their local dbts

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.42% 🎉

Comparison is base (9c8a9d9) 77.95% compared to head (f90843a) 82.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
+ Coverage   77.95%   82.38%   +4.42%     
==========================================
  Files          45       46       +1     
  Lines        1901     1839      -62     
==========================================
+ Hits         1482     1515      +33     
+ Misses        419      324      -95     

Files Changed	Coverage Δ
ddpui/celeryworkers/tasks.py	25.71% <ø> (+11.74%)	⬆️
.../0023_remove_orgdbt_dbt_version_orgdbt_dbt_venv.py	100.00% <100.00%> (ø)
ddpui/models/org.py	92.30% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[fatchat]

OrgDbt.objects.filter(dbt_venv__isnull=True).update(dbt_venv="/home/ddp/dbt")