Add CTAs to daily emails

Add a daily email CTA node type and reference field to attach a CTA to a
daily email.

Also update the RSS feed view to include the referenced body field
value.

config/sync/field.storage.node.field_daily_email_cta.yml

@@ -0,0 +1,19 @@
+uuid: 1df230c0-e769-4208-88a7-19c756975886
+langcode: en
+status: true
+dependencies:
+  module:
+    - node
+id: node.field_daily_email_cta
+field_name: field_daily_email_cta
+entity_type: node
+type: entity_reference
+settings:
+  target_type: node
+module: core
+locked: false
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false