Add the Flutter app that will be migrated from SPA.

[daisukenishino2]

Requirement

• Add the Flutter app that will be migrated from SPA.
• See Add the CRUD sample to Vue template. #2 for the telegram of process that to be implemented.

[daisukenishino2]

• Added flutter_template.
  9c1fabe
• Fixed the folder structure.
  b41d0aa
• Splitting the widget.
  269bab5
• Fixed the part that was bothering.
  18e19ef
• Prepared for CRUD implementation.
  52ffec7
• Prepared for CRUD implementation.
  75e8042
• Prepared for CRUD implementation.
  a121b2b
• Implemented the reference system.
  ae2feac
• Implemented the CRUD processes.
  ebb63d4
• Use access token.
  2240371