Move TimeSpanConverter with another Converters

Follow indications :
https://github.com/iv-org/invidious/pull/2915#discussion_r811373953

src/invidious/user/preferences.cr

@@ -256,4 +256,18 @@ struct Preferences
       cookies
     end
   end
-end
+
+  module TimeSpanConverter
+    def self.to_yaml(value : Time::Span, yaml : YAML::Nodes::Builder)
+      return yaml.scalar recode_length_seconds(value.total_seconds.to_i32)
+    end
+
+    def self.from_yaml(ctx : YAML::ParseContext, node : YAML::Nodes::Node) : Time::Span
+      if node.is_a?(YAML::Nodes::Scalar)
+        return decode_time_span(node.value)
+      else
+        node.raise "Expected scalar, not #{node.class}"
+      end
+    end
+  end
+end