Add district_name to collection (#639)

## Status

- Closes [Admin Console: Add 'District Name' Column to Schools
List](RaspberryPiFoundation/digital-editor-issues#1051)

## What's changed?
 
- Added `distict_name` field to the schools list on the admin dashboard

Author: jamiebenstead

app/dashboards/school_dashboard.rb

@@ -47,6 +47,7 @@ class SchoolDashboard < Administrate::BaseDashboard
     reference
     country_code
     administrative_area
+    district_name
     created_at
     verified_at
     rejected_at