Show all selected labels at top of list

What does this MR do and why?

Fix work item Selected labels for big collections.

In groups with more than 100 labels, it was possible that some were not displayed

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Create lots of labels. This makes 1000 for the gitlab-org group

   group = Group.first  # or Group.find(123)
   
   1000.times do |i|
     name = "Label #{i + 1}"
     label = group.labels.create!(
       name: name,
       color: "#%06x" % (rand * 0xffffff) # random hex color
     )
   end

2. Load a work item in that group
3. Add some labels