Account::Sync model and test fixture simplifications (#968)

* Add sync model

* Fresh fixtures for sync tests

* Sync tests overhaul

* Fix entry tests

* Complete remaining model test updates

* Update system tests

* Update demo data task

* Add system tests back to PR checks

* More simplifications, add empty family to fixtures for easier testing

test/models/category_test.rb

@@ -20,7 +20,7 @@ class CategoryTest < ActiveSupport::TestCase
   end
 
   test "updating name should clear the internal_category field" do
-    category = Category.take
+    category = categories(:income)
     assert_changes "category.reload.internal_category", to: nil do
       category.update_attribute(:name, "new name")
     end