users: set default settings in ruby instead of in database.

Specify the default settings for new users inside the User model instead
of inside the database. This makes it easier to change defaults, and it
makes the code clearer.

test/functional/forum_topics_controller_test.rb

@@ -72,7 +72,7 @@ class ForumTopicsControllerTest < ActionDispatch::IntegrationTest
       should "not record a topic visit for non-html requests" do
         get_auth forum_topic_path(@forum_topic), @user, params: {format: :json}
         @user.reload
-        assert_nil(@user.last_forum_read_at)
+        assert_equal(Time.zone.parse("1960-01-01"), @user.last_forum_read_at)
       end
 
       should "render for atom feed" do