use custom_preview_path env variable for mailer previews 

https://github.com/compassionprojects/socialchange/commit/3411abba08c8e276823ccb1a599c76b675df89cb#diff-d3c4b3f41072daa416f1920511e9b2e26caea8c5cec0a14cb9508589a4dafa47

https://github.com/rspec/rspec-rails/commit/0dbdd6fad21e43db8c53f525c01135b38033a19f