Use integer columns for ImageServer and TranscriptionEntity IDs in SQLIte export

The image_server.id, image.server_id and transcription_entity.id columns in the SQLite export are defined as VARCHAR(37) when they actually are integers in the PostgreSQL database, since those fields were never explicitly set to something other than Django's default AutoField. We should redefine them as integers to stay consistent and avoid confusion as users may choose to parse them as UUIDs.

This requires bumping the export version, updating the docs, and updating the export library.