Simplify code to find a content block

We're only calling the `block_for` method from one place: the
`content_block` helper, and we're never passing the locale parameter to
that helper, which means we're always calling `block_for` using
`I18n.locale`.

So I'm not sure why we were doing the `locale ||=` assignment, since
`I18n.locale` doesn't return nil.

In any case, since this was added in commit c1de2dced, Ruby has added
support for arguments forwarding, so we can use it here to simplify the
code a little bit.

app/helpers/application_helper.rb

@@ -46,8 +46,8 @@ module ApplicationHelper
     end
   end
 
-  def content_block(name, locale = I18n.locale)
-    SiteCustomization::ContentBlock.block_for(name, locale)
+  def content_block(...)
+    SiteCustomization::ContentBlock.block_for(...)
   end
 
   def self.asset_data_base64(path)