docker: add postfix email support for ActionMailer

[mcritchlow]

Fixes #319

Local Checklist

• [x] Tests written and passing locally?
• [x] Code style checked?
• [x] QA-ed locally?
• [x] Rebased with master branch?
• [ ] Configuration updated (if needed)?
• [ ] Documentation updated (if needed)?

What does this PR do?

• Adds postfix to the development and production docker images
• Adds a postfix start command to the docker entrypoint script
• Split the ldap data loading out of the seeds.rb file, as this cause a catastrophe that I am still sad about
• Add the ldap employee data population to the docker entrypoint script. This will allow us to spin up a new container and have all the data we need populated
• Add support in the docker entrypoint script for using the production database host environment variable instead of DATABASE_URL. This is some early support for using the container image for real deploys. More work is needed here, but can be a separate ticket.

Why are we doing this? Any context of related work?

References #319

The container image for high five needs to support all the required features that the current application has sitting on a VM. This adds email support for ActionMailer.

@ucsdlib/developers - please review

[coveralls]

Coverage remained the same at 99.66% when pulling 55ebc2e191c040b24c3520ebbb17f1b6cab038d3 on docker-postfix into 52c3545cc71a09abca573b720d67399f67f10c54 on master.