FEATURE: Adds a pop up that shows a more detailed score for reviewables (#8035)

If you click a (?) icon beside the reviewable status a pop up will
appear with expanded informatio that explains how the reviewable got its
score, and how it compares to system thresholds.

app/controllers/reviewables_controller.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+require_dependency 'reviewable_explanation_serializer'
 
 class ReviewablesController < ApplicationController
   requires_login
@@ -102,6 +103,17 @@ class ReviewablesController < ApplicationController
     )
   end
 
+  def explain
+    reviewable = find_reviewable
+
+    render_serialized(
+      { reviewable: reviewable, scores: reviewable.explain_score },
+      ReviewableExplanationSerializer,
+      rest_serializer: true,
+      root: 'reviewable_explanation'
+    )
+  end
+
   def show
     reviewable = find_reviewable