Refactor unnesting for service alerts

[lauriemerrell]

Description

Describe your changes and why you're making them. Please include the context, motivation, and relevant dependencies.

In the course of working on #2489, I realized that our current unnesting for service alerts does not seem appropriate. We have three separate tables for informed entities, active periods, and translations, but an individual "alert" actually comprises all three of these elements.

It makes more sense to have one fully unnested table, deduped on language.

Resolves #N/A

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

How has this been tested?

Include commands/logs/screenshots as relevant.

All new tests pass on the new tables. I also did full refreshes of the two updated mart tables (daily and trip_summaries) and confirmed that for the past week my namespace copy is identical to the prod copy:

WITH prod AS (
  SELECT *
  FROM `cal-itp-data-infra.mart_gtfs.fct_service_alerts_trip_summaries`
),

tst AS (
  SELECT *
  FROM `cal-itp-data-infra-staging.laurie_mart_gtfs.fct_service_alerts_trip_summaries`
)

SELECT 
  prod.*,
  tst.*
FROM prod
FULL OUTER JOIN tst
USING (key)
WHERE COALESCE(prod.dt, tst.dt) > '2023-05-04' AND COALESCE(prod.dt, tst.dt) < CURRENT_DATE() AND (prod.key IS NULL OR tst.key IS NULL)

Post-merge follow-ups

Document any actions that must be taken post-merge to deploy or otherwise implement the changes in this PR (for example, running a full refresh of some incremental model in dbt). If these actions will take more than a few hours after the merge or if they will be completed by someone other than the PR author, please create a dedicated follow-up issue and link it here to track resolution.

• No action required
• Actions required (specified below)

Finish the deletion process for the deprecated mart tables; they have been added to the deprecation spreadsheet and have been announced on Slack. I have reminders set to soft- and hard-delete in 1 and 2 weeks respectively.

No full refresh is required after merge because the swap-in is identical and the partitions are the same.

[github-actions]

Warehouse report 📦

New models 🌱

mart.gtfs.fct_service_alerts_messages_unnested
intermediate.gtfs.int_gtfs_rt__service_alerts_fully_unnested

Changed models 🔀

mart.gtfs.fct_daily_service_alerts
mart.gtfs.fct_service_alerts_trip_summaries

[atvaccaro]

lgtm thanks for fixing my stuff