Move the class configuration defaults into Blacklight::Configuration.default_values

[cbeer]

And, now that we've dropped Rails 5.x, we can use #deep_transform_values directly.

[jcoyne]

@cbeer there is a conflict here that needs to be resolved.

[dkinzer]

Is there a way to keep track of these backwards incompatible changes to make the upgrade path easier?

[cbeer]

These aren't backwards incompatible changes, just moving where/how the values are set initially.

[dkinzer]

@cbeer Yeah, sorry. I was focusing on the signature of the deep_transform_values method changing without taking into account that's a private mehod.