Rename `Gitlab::CurrentSettings` to `AppConfig::Settings`

Problem

With the addition of AppConfig bounded context, the Gitlab::CurrentSettings is not compliant.

Proposal

Rename Gitlab::CurrentSettings to AppConfig::Settings

AppConfig::Settings # => read-only model
AppConfig::Settings.deny_all_requests_except_allowed? # => recommended - read single setting (internally will use `all_settings`)
AppConfig::Settings.all_settings # => returns AR read-only model from cache. (this is to be used only exceptionally)
AppConfig::Settings.all_uncached_settings # => returns AR read-only model straight from the database. (this is to be used only exceptionally)