Add to close user account

[dtslvr]

A user should be able to close their account.

Tasks

• Add new endpoint DELETE api/v1/user (without :id) with accessToken: string and validate it
•  Introduce new permission deleteOwnUser and assign it to the USER role
• Extend UserAccountSettingsComponent by a form if permission deleteOwnUser is present and let the user confirm this action
  confirm($localize`Do you really want to close your account?`);