Implement Document Settings UI for storage options for attachments

[dsagal]

The externalized attachments project (design doc here) allows each document to select whether attachments are stored internally in the doc, or in a preconfigured external storage.

The switch would be done in two steps:

1. Set the desired destination/storage for attachments; new attachments will get stored there (just two options for now, "internal" and "external").
2. Start the transfer of existing attachments to the selected destination. This may take some time.

Because of the two-step process and the fact that the transfer may take time, the current storage could be "internal", "external", or "mixed".

[lusebille]

Mocks are here (upper part of the board) https://www.figma.com/design/wcpetFt6aOKzTszcvPPWLQ/%5B05%2F24%5D-Grist-Design?node-id=3600-82240&t=kFXwKO4RsotbUKLw-1

[dsagal]

There is a new design (apologies for the late change): https://www.figma.com/design/wcpetFt6aOKzTszcvPPWLQ/%5B05%2F24%5D-Grist-Design?node-id=3600-82240&t=PYGb5S1L0MQJm1qz-0. I think it's better, because in the common case the user will just see a single clear setting.

The exact text needs review. I'll leave some comments in Figma, and @Spoffy perhaps you can review too, so that it matches how we are planning to talk about this feature elsewhere.