Import/export project format

Fixes #70 (closed)

Import and export projects from Taguette SQLite3 files.

• Implement database copy routine
• Import
  • Add API endpoint to list projects in a database file
  • Add API endpoint to import project from database file
  • Add import form to provide a database and select a project to import
  • Add test
• Export
  • Add export endpoint to get a database with a given project
  • Add test