Allow administrators to become a different user by adding an impersonate action
to the user list. This is especially useful for debugging issues reported by
users.

To impersonate a user an administrator has to simply follow the following four
steps:

1. Login as administrator to Nextcloud.
2. Open users administration interface.
3. Select the impersonate button on the affected user.
4. Confirm the impersonation.

The administrator is then logged-in as the user, to switch back to the regular
user account they simply have to press the logout button.