Clears the current Appearance cache otherwise it breaks since...

Clears the current Appearance cache otherwise it breaks since new_project_guidelines_html would be missing

Solves https://gitlab.com/gitlab-org/gitlab-ce/issues/41041.
Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20171122131600_add_new_project_guidelines_to_appearances.rb

@@ -4,6 +4,12 @@ class AddNewProjectGuidelinesToAppearances < ActiveRecord::Migration
   DOWNTIME = false
 
   def change
+    # Clears the current Appearance cache otherwise it breaks since
+    # new_project_guidelines_html would be missing. See
+    # https://gitlab.com/gitlab-org/gitlab-ce/issues/41041
+    # We're not using Appearance#flush_redis_cache on purpose here.
+    Rails.cache.delete('current_appearance')
+
     change_table :appearances do |t|
       t.text :new_project_guidelines
       t.text :new_project_guidelines_html