FEATURE: allow customizing default timezone for email. (#32964)

1. Changes default email format to llll (eg: Tue, May 8, 2018 2:00 AM)
from 2018-05-08T00:00:00Z UTC which is not nice on the eyes
2. Adds `discourse_local_dates_email_timezone` which allows configuring
default timezone in emails
3. Improved help text on site settings (format / timezone)

---------

Co-authored-by: Gary Pendergast <gary@pento.net>

plugins/discourse-local-dates/db/migrate/20250529021721_add_utc_to_setting.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+class AddUtcToSetting < ActiveRecord::Migration[7.2]
+  def up
+    # we changed the setting so UTC is no longer appended, we append it now in the format
+    execute <<~SQL
+      UPDATE site_settings
+      SET value = value || ' z'
+      WHERE name = 'discourse_local_dates_email_format'
+    SQL
+  end
+
+  def down
+    execute <<~SQL
+      UPDATE site_settings
+      SET value = REPLACE(value, ' z', '')
+      WHERE name = 'discourse_local_dates_email_format'
+    SQL
+  end
+end