Add deactivates_at date to user blocks

Block deactivation dates that take needs_view-block views into account were derived using updated_at. This was possible because inactive blocks couldn't be edited and their updated_at date wouldn't change. With editing of inactive blocks enabled deactivation date needs to be saved explicitly.

test/factories/user_blocks.rb

@@ -2,12 +2,14 @@ FactoryBot.define do
   factory :user_block do
     sequence(:reason) { |n| "User Block #{n}" }
     ends_at { Time.now.utc + 1.day }
+    deactivates_at { ends_at }
 
     user
     creator :factory => :moderator_user
 
     trait :needs_view do
       needs_view { true }
+      deactivates_at { nil }
     end
 
     trait :expired do