[Story] delete users

[halsk]

Goal:

As a registered user, I want to delete my account so that I can quit using this website.

This issue is related to #5

Tasks:

1. Consider how we handle the projects and activities associated with the deleting user.
2. Implements the features

Acceptance Criteria:

• Criteria 1.
• Criteria 2.
• ...

[NEKOYASAN]

I think there are three patterns of processing when deleting users in Projects and Activities.

1. Delete all projects and activities related to that user. (≒ physical deletion)
2. Replace the user name with a unique name, such as a user ID, and retain the projects and activities associated with the deleted user.
3. Projects associated with a user will be transferred to another user or deleted, and activities will be displayed as deleted or deleted users. (Require that there are no projects associated with the user when deleting.)

[MaySoMusician]

We can force the project owners to transfer the ownerships of their projects to someone before they request to delete their account. Discord takes the same approach, where the users are unable to delete their accounts unless they transfer all of the server ownerships to the others (or delete the servers first).