fix(aci): Rework logic to backfill open periods

[snigdhas]

This PR changes the backfill logic to make sure it works with the regressed/auto-resolved cycles that tripped us up earlier. There's also a handful of cases where we see successive regressions / resolutions and the old logic didn't know how to handle that. These changes will set the open period based on the first regression/first resolution if there are multiple in a row.

Makes for an easier review if you compare against the first commit which just copies the changes from migration 0878

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0915_backfill_open_periods.py

for 0915_backfill_open_periods in sentry

--
-- Raw Python operation
--
-- THIS OPERATION CANNOT BE WRITTEN AS SQL

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0924_backfill_open_periods.py

for 0924_backfill_open_periods in sentry

--
-- Raw Python operation
--
-- THIS OPERATION CANNOT BE WRITTEN AS SQL

[wedamija]

The migration lgtm, and the logic changes seem to make sense, but I don't have too much context so it'd be good to have someone from your team confirm as well if they have more context