Merged PR 4471: Visualisation changes

# Description

Adapts YTD/MTD for visualisation purposes

# Checklist

- [X] The edited models and dependants run properly with production data.
- [X] The edited models are sufficiently documented.
- [X] The edited models contain PK tests, and I've ran and passed them.
- [X] I have checked for DRY opportunities with other models and docs.
- [X] I've picked the right materialization for the affected models.

# Other

- [ ] Check if a full-refresh is required after this PR is merged.

Visualisation changes

Related work items: #27609

models/reporting/general/schema.yml

@@ -1968,6 +1968,12 @@ models:
         data_tests:
           - not_null
 
+      - name: first_day_month
+        data_type: date
+        description: The first day of the month associated with the data.
+        data_tests:
+          - not_null
+
       - name: calendar_year
         data_type: integer
         description: The calendar year associated with the data.