Include relative url configuration in rake tasks

initializers are not loaded during the rake task assets:precompile,
so assets generation runs without relative url configuration in Rails
and produces broken assets.

CHANGELOG

@@ -10,6 +10,7 @@ v 8.6.0 (unreleased)
 v 8.5.2
   - Fix sidebar overlapping content when screen width was below 1200px
   - Fix error 500 when commenting on a commit
+  - Fix broken icons on installations with relative URL (Artem Sidorenko)
 
 v 8.5.1
   - Fix group projects styles

Rakefile

@@ -4,4 +4,7 @@
 
 require File.expand_path('../config/application', __FILE__)
 
+relative_url_conf = File.expand_path('../config/initializers/relative_url', __FILE__)
+require relative_url_conf if File.exist?("#{relative_url_conf}.rb")
+
 Gitlab::Application.load_tasks