Handle assignments in an ActiveModel::Dirty-friendly way

[mshibuya]

This is a proposal for solving ActiveModel::Dirty related issues.
Previously saving an attachment takes following process:

• On attachment assignment: does not assign the attribute value but call #{column}_will_change!
• Before save: assign identifier as the attribute value

After this PR, it will be like:

• On attachment assignment: assign temporary identifier as the attribute value (this marks the column as changed, so we don't need to call #{column}_will_change!)
• Before save: assign the true identifier as the attribute value

The reason for introducing the idea of temporary identifier is that there are the cases we can't determine the identifier on the attachment assignment - an identifier is made from the filename, but filename can be overridden to reference another attribute in the model and in this case we can't determine the final filename until the record is saved (i.e. that column can be assigned after the attachment assignment).

Doing this allows us to reflect #changes correctly, resulting in fixing #2404. Also not needing to call #{column}_will_change! fixes #2409 and #2468, as empty changes will no longer be picked up.

Any feedback would be appreciated, thanks.