Adapt to new DropBox API

[koba-e964]

Description / 説明

DropBox API changed on Sep 30, 2021. Since then, long access tokens are not granted.
We have to first obtain AUTHORIZATION_CODE (from Web UI), and after that obtain ACCESS_TOKEN from AUTHORIZATION CODE (and <APP_KEY>:<APP_SECRET>).

The generated ACCESS_TOKEN will expire in 14400 secs = 4 hrs. After that, we must regenerate ACCESS_TOKEN via refresh_token.

For compatibility reason, we should keep the old code as-is.

Motivation / 動機

The current DropBox App cannot have new users, presumably because of the limit of the number of users.
We are going to develop a new one, hopefully with no limits on the number of users.

Other notes / その他

• Will you try to create a pull request?
  • yes