Decouple from Rails secrets

[vr4b4c]

Task: #78

Aim

Remove dependency on Rails secrets and let clients choose how to manage secret parameters.

Initialization

Devise omniauth initialization procedure is moved from Rails initializer to Railtie initializer because it supports more granular control over initialization order. Rails config initializer for dependencies runs before host applications config initializers. This is a problem because Devise omniauth is configured before the host can set infinum_azure configuration options (eg. load params from configuration and/or secret stores). infinum_azure initialization should run after the host application config initializer and before devise.omniauth initializer, which can be expressed via the Raitie initializer.

Fixes #3

Solution

• Remove dependency on Rails secrets
• Remove Defaults module and refactor Config
• Remove service_name property (not used)
• Update README and CHANGELOG