ref: have models.User extend Model instead of BaseModel

[asottile-sentry]

most of our db helpers expected .id to be a Field[int, int] and use Model as the type for that

no migration was generated so I believe this to be a fancy noop

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.05%. Comparing base (4cdd6f9) to head (b3aa840).
Report is 18 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #72967   +/-   ##
=======================================
  Coverage   78.05%   78.05%           
=======================================
  Files        6610     6610           
  Lines      294851   294866   +15     
  Branches    50811    50817    +6     
=======================================
+ Hits       230144   230159   +15     
+ Misses      58369    58367    -2     
- Partials     6338     6340    +2     

Files	Coverage Δ
src/sentry/api/bases/user.py	85.39% <100.00%> (ø)
src/sentry/api/endpoints/api_tokens.py	100.00% <100.00%> (ø)
src/sentry/models/user.py	90.39% <100.00%> (-0.04%)	⬇️
src/sentry/tasks/summaries/weekly_reports.py	85.86% <100.00%> (ø)

... and 8 files with indirect coverage changes

[markstory]

No migration makes sense to me as there weren't any relevant changes to django model state or the schema.