ref: move notifications models to a separate django app

[asottile-sentry]

this breaks the migration cycle between sentry<=>workflow_engine

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0927_move_notifications_models.py src/sentry/notifications/migrations/0001_move_notifications_models.py

for 0927_move_notifications_models in sentry

--
-- Custom state/database change combination
--
-- (no-op)

for 0001_move_notifications_models in notifications

--
-- Custom state/database change combination
--
-- (no-op)

[asottile-sentry]

note that the migrations here are state-only -- I also validated with #93557 that they produce no drift \o/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #93582      +/-   ##
==========================================
- Coverage   88.01%   88.01%   -0.01%     
==========================================
  Files       10322    10324       +2     
  Lines      595057   595079      +22     
  Branches    23125    23125              
==========================================
+ Hits       523725   523737      +12     
- Misses      70839    70849      +10     
  Partials      493      493              

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0928_move_notifications_models.py src/sentry/notifications/migrations/0001_move_notifications_models.py

for 0928_move_notifications_models in sentry

--
-- Custom state/database change combination
--
-- (no-op)

for 0001_move_notifications_models in notifications

--
-- Custom state/database change combination
--
-- (no-op)