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.

spec/models/custom_emoji_spec.rb

@@ -75,4 +75,13 @@ RSpec.describe CustomEmoji, type: :model do
       end
     end
   end
+
+  describe 'pre_validation' do
+    let(:custom_emoji) { Fabricate(:custom_emoji, domain: 'wWw.MaStOdOn.CoM') }
+
+    it 'should downcase' do
+      custom_emoji.valid?
+      expect(custom_emoji.domain).to eq('www.mastodon.com')
+    end
+  end
 end