Make custom emoji domains case insensitive #9351 (#9474)

* Make custom emoji domains case sensitive #9351

* Fixup style in downcase_domain to comply with codeclimate.

* switch if! to unless

* Don't use transactions, operate in batches.

Also revert spurious schema change.

db/migrate/20181207011115_downcase_custom_emoji_domains.rb

@@ -0,0 +1,7 @@
+class DowncaseCustomEmojiDomains < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def change
+    CustomEmoji.in_batches.update_all('domain = lower(domain)')
+  end
+end