Adds candidate count to model detail

What does this MR do and why?

Adds candidate count to model detail

Displays the count of candidates that are not part of a model version

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:model_registry)

2. Create a model and a bunch of candidates

   p = Project.find_by_id(1)
   m = Ml::FindOrCreateModelService.new(p, "model_1").execute
   5.times { |i| Ml::CreateCandidateService.new(m.default_experiment, {}).execute }

3. Navigate to <gdk>/<project>/-/ml/models and select the model. Verify that the number by Version candidates is correct

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.