Add default email server setup

[RickMohr]

People setting up OTM can have trouble configuring their email server. That's not part of OTM, but having a default setup that works would smooth the road. Something like the following (based on our Treezilla provision script and written for an otm-core deployer) should suffice (plus instructions to set SUPPORT_EMAIL_ADDRESS and DEFAULT_FROM_EMAIL). And we should make corresponding updates to the installation wiki page.

1. Add to local_settings.py:

EMAIL_BACKEND = 'django.core.mail.backends.smtp.EmailBackend'
EMAIL_HOST = 'localhost'
EMAIL_PORT = 25
EMAIL_HOST_USER = ''
EMAIL_HOST_PASSWORD = ''
EMAIL_USE_TLS = False
SUPPORT_EMAIL_ADDRESS = 'support@yoursite.com'
DEFAULT_FROM_EMAIL = '<email-address-you-want-in-the-from-line@yourdomain.com>'

2. Install the postfix email server:

sudo DEBIAN_FRONTEND=noninteractive apt-get install postfix

3. Restart the Django app:

sudo service otm-unicorn restart

4. Register a new user and see if you get an email