[tenantadm] add account cancellation endpoint

On calling this endpoint a canned email should be sent to support@mender.io using the corresponding workflows service (send_email worker).

Acceptance criteria:

• the endpoint accepts an account cancellation request
  • the request should allow for a cancellation reason to be set
• the sent email should contain:
  • tenantid
  • user id & user email of the cancelling account
  • cancellation reason
• tests exist & pass
  • especially: the call to the workflows service is tested