Allow mutators for Settings models and improve API

[bennothommo]

Fixes #1047. Replaces #1071.

• Use getAttribute method in the model.setAttribute listener within the SettingsModel behavior to retrieve the (possibly) mutated value before saving into settings array.
• Ensure that default values populated by initSettingsData method in a Settings model are populated into settings array if not yet persisted in the DB, so that they are accessible via model properties.
• Allow values to be retrieved as model properties at any time.

[bennothommo]

@goldmont would you be willing to test this PR as an alternate fix for #1047? I've added a test case for your scenario, but happy to hear if it works out in the wild.