Add Send to social media checkbox to posts

Also add it to the `opdavies_blog_test` module config so that the
existing tests continue to work and pass.

References #249

config/default/field.storage.node.field_send_to_social_media.yml

@@ -0,0 +1,18 @@
+uuid: a1c878f4-98c2-4a10-8f80-adc48186b0f6
+langcode: en
+status: true
+dependencies:
+  module:
+    - node
+id: node.field_send_to_social_media
+field_name: field_send_to_social_media
+entity_type: node
+type: boolean
+settings: {  }
+module: core
+locked: false
+cardinality: 1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false