Fix inconsistent label foreground color

In different parts of the application, the same label was being rendered
with a different foreground color. This was due to the "textColor" JSON
property not being generated by the backend. This commit adds
this property to JSON representation of labels.

app/models/label.rb

@@ -214,6 +214,7 @@ class Label < ActiveRecord::Base
     super(options).tap do |json|
       json[:type] = self.try(:type)
       json[:priority] = priority(options[:project]) if options.key?(:project)
+      json[:textColor] = text_color
     end
   end
 

changelogs/unreleased/53714-inconsistent-text-color-for-labels.yml

+---
+title: Fix foreground color for labels to ensure consistency of label appearance
+merge_request: 23873
+author: Nathan Friend
+type: fixed