Undefined selector JavaScript error after merging when super sidebar enabled

This was a red herring for #407833 (closed), but I'm logging this in case this is an issue that hasn't been noticed. I reproduced the issue in #407833 (comment 1357946412) that resulted in this JavaScript error:

It seems that refreshUserMergeRequestCounts() fails because .js-assigned-mr-count doesn't exist when the new sidebar is enabled.

To reproduce:

1. In the Rails console, run:

   Feature.enable(:super_sidebar_nav)
   user = User.first
   user.update!(use_new_navigation: true)

2. Create a new merge request.

3. Click Merge.

/cc: @pgascouvaillancourt