Add a preferences API so apps can share basic behaviours (#10109)

app/serializers/rest/preferences_serializer.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class REST::PreferencesSerializer < ActiveModel::Serializer
+  attribute :posting_default_privacy, key: 'posting:default:visibility'
+  attribute :posting_default_sensitive, key: 'posting:default:sensitive'
+  attribute :posting_default_language, key: 'posting:default:language'
+
+  attribute :reading_default_sensitive_media, key: 'reading:expand:media'
+  attribute :reading_default_sensitive_text, key: 'reading:expand:spoilers'
+
+  def posting_default_privacy
+    object.user.setting_default_privacy
+  end
+
+  def posting_default_sensitive
+    object.user.setting_default_sensitive
+  end
+
+  def posting_default_language
+    object.user.setting_default_language.presence
+  end
+
+  def reading_default_sensitive_media
+    object.user.setting_display_media
+  end
+
+  def reading_default_sensitive_text
+    object.user.setting_expand_spoilers
+  end
+end