Stop using Env vars for config/settings

[aliuk2012]

We need to document our environment variables and centralise our various settings. I would like to propose that we use config gem which creates a yaml file for various environment (like .env) files.

Benefits:

• can still use environment vars to override settings
• can namespace different config/settings
• add inline comments relating to want each config/setting is for
• it is possible to "refresh" the settings without restarting the servers using Settings.reload!

When we start needing to us feature flags we can also use these yaml files to store the flags in.

example:

features:
   show_preview_links: true

Refer to alphagov/forms-admin#118 as an example