CORE-4617 Refactored email to full_username in TemplateServiceFacade.

Updated the TemplateServiceFacade#deleteAppFromWorkspace to submit the 'full_username' field in its JSON request, rather than 'email', to convey the actual intent of this field's value.
The service endpoint looks for the username in either field, and since this client is sending the full username, this JSON field should match, especially since a user may have a different email than their fully qualified username.