Improve the configuration model

[Nubbify]

What we're doing here is both adding a new dictionary for default values (in anticipation of pre-populating these fields in the new config designs and also because it's a bit cleaner) and also changing the way we do validation so that we return useful error messages in our errors array whenever validation fails.

This pull request makes the following changes:

• New DEFAULTS dictionary inside of the config model
• Return validation failure reasons from config controllers

It relates to the following issue #s:

• UI cleanup work on Config index page #1559