Allow GoodJob global configuration accessors to also be set via Rails config hash

[bensheldon]

Deprecates all GoodJob configuration accessors, in favor of Rails configuration hash.

I was originally planning to deprecate them now, but I plan with GoodJob 3.0 to change them as a result of #412 so I'll deprecate most of them (with the exception of logger and on_thread_error) when I do that.

• Creates a callable GoodJob._on_thread_error method
• Wraps all of the Railtie code initializer blocks
• Fixes some app configuration from Defer async initialization until Rails fully initialized #454

Connects to #380.