This commit attempts to better organize ColdFront settings and
configuration. As the number of plugins grows having a well defined way
to configure the ColdFront application will be imperative. The main
goals are:

1. Split the config/settings across multiple files for better
organization.

2. Allow configuring ColdFront from environment variables or a env file

3. Enable easy way to set custom config values system wide for
production installs as well as dev/staging environments

In this commit we use django-environ to fetch config settings from the
environment. We also use django-split-settings which provides a way to
split up django settings across multiple files.

The organization is as follows:

- coldfront/config/base.py = Base Django config settings
- coldfront/config/auth.py = Authenication backends
- coldfront/config/database.py = Database backends
- coldfront/config/email.py = Email settings
- coldfront/config/logging.py = Logging settings
- coldfront/config/core.py = Core ColdFront settings
- coldfront/config/plugins/* = ColdFront Plugins

Sites can override any config settings by creating a local_settings.py
file in coldfront/config, or /etc/coldfront/local_settings.py.
Additionally, they can override settings using environment variables, or
an environment file. ColdFront will search for environment files in this
order: .env (in current working directory) or
/etc/coldfront/coldfront.env. You can also specify the path to the
environment file using COLDFRONT_ENV.