Re-allow appearances.description_html to be NULL

This column isn't always set (e.g. when upgrading from older instances)
and technically it could be NULL (e.g. when flushing the cache).

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36919

changelogs/unreleased/revert-appearances-description-html-not-null.yml

+---
+title: Re-allow appearances.description_html to be NULL
+merge_request:
+author:
+type: fixed

db/migrate/20170809142252_cleanup_appearances_schema.rb

@@ -7,7 +7,7 @@ class CleanupAppearancesSchema < ActiveRecord::Migration
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
-  NOT_NULL_COLUMNS = %i[title description description_html created_at updated_at]
+  NOT_NULL_COLUMNS = %i[title description created_at updated_at]
 
   TIME_COLUMNS = %i[created_at updated_at]
 

db/migrate/20170824162758_allow_appearances_description_html_null.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AllowAppearancesDescriptionHtmlNull < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def up
+    change_column_null :appearances, :description_html, true
+  end
+
+  def down
+    # This column should not have a `NOT NULL` class, so we don't want to revert
+    # back to re-adding it.
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170820100558) do
+ActiveRecord::Schema.define(version: 20170824162758) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -34,7 +34,7 @@ ActiveRecord::Schema.define(version: 20170820100558) do
     t.string "logo"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.text "description_html", null: false
+    t.text "description_html"
     t.integer "cached_markdown_version"
   end