[BIS2036] update mailer_domain on staging

we have staging on `/backend`, just like production.

[podio story](https://podio.com/bitzesty/bis-phase-2/apps/stories/items/38)
[BIS2036] force staging and production to use ssl
[BIS2036] use `https` for mailers on production & staging
Merge pull request #39 from bitzesty/BIS2036-mailer-host

[BIS2036] update mailer_domain on staging