This allows deprecation messages to be matched by substring, symbol (treated as
substring), or regular expression. If a warning is matched, the behaviors
configured for disallowed deprecations will be used. The default behavior for
disallowed deprecation warnings is `:raise`.

Also adds `ActiveSupport::Deprecation.allow` for thread-local, block level ignoring of deprecation warnings which would otherwise be disallowed by ActiveSupport::Deprecation.disallowed_warnings.