FIX: Delete associated channel upon category deletion

Currently when a category is deleted, if it has an associated chat
channel, the latter won’t be deleted automatically.

The fix is quite simple as we were simply missing a `dependent:
:destroy` option on the existing relation.

plugins/chat/spec/models/category_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Category do
     let(:channel_class) { CategoryChannel }
   end
 
-  it { is_expected.to have_one(:category_channel) }
+  it { is_expected.to have_one(:category_channel).dependent(:destroy) }
 
   describe "#cannot_delete_reason" do
     subject(:reason) { category.cannot_delete_reason }