OCS API endpoint to resend welcome message

* send a POST request to ocs/v1.php/cloud/users/USERNAME/resendWelcomeMessage to trigger
  the welcome message to be send
* fixes #3367

example curl statement:

  curl -i https://example.org/ocs/v1.php/cloud/users/USERNAME/welcome

 -H  "OCS-APIRequest: true" -u admin:password -X POST

Signed-off-by: Morris Jobke <hey@morrisjobke.de>