Minor import export refactoring

Just a little bit of refactoring clean up, removing some unnecessary methods and changing class names.

I decided to make the Importer classes services, it seems like it better suits what they are doing and where they are being used. Since I'm also working on the automate import, which I will also be calling a service, I wanted to change these as well.
It just ensures we know what error we're raising. If something else gets raised, it'll blow up here and the stacktrace will show that it blew up here. If ```DialogFieldImporter::InvalidDialogFieldTypeError``` gets raised, then we'll re-raise it.

I suppose it's not necessary, but with specific errors, I like to be explicit and then the things that use the ```DialogImportService``` should then know that they could potentially need to handle this error.