Lazily load the attachment_upload_to configuration setting

[djw27]

In our project structure we can't simply import a function into our settings file to use within the SUMMERNOTE_CONFIG - trying to import a function before the project has been initialised simply doesn't work. In our specific case we import settings from environment variables and use a multiple settings files per environment which complicates things further.

This PR changes the attachment_upload_to to accept the dot notation to the file to be used, which is then loaded when it's needed.

This is a breaking change so I could work some backwards compatibility in to this if it's something you'd like to use in master?

[coveralls]

Coverage decreased (-1.9%) to 96.656% when pulling 0323284 on workshoptech:master into f5f34de on summernote:master.

[coveralls]

Coverage decreased (-1.9%) to 96.644% when pulling 6f34790 on workshoptech:master into 5bcd829 on summernote:master.

[lqez]

Thank you for contribution, @djw27.

We can make it without breaking backward compatibilties by detecting the type of attachment_upload_to. Could you make it uses get_by_qname only when a string has given to the config?

And it would be much appreciated if you can also provide a test for it.
Thanks.