Add option to be notified when a followed user posts (#13546)

* Add bell button

Fix #4890

* Remove duplicate type from post-deployment migration

* Fix legacy class type mappings

* Improve query performance with better index

* Fix validation

* Remove redundant index from notifications

spec/models/concerns/account_interactions_spec.rb

@@ -14,7 +14,7 @@ describe AccountInteractions do
     context 'account with Follow' do
       it 'returns { target_account_id => true }' do
         Fabricate(:follow, account: account, target_account: target_account)
-        is_expected.to eq(target_account_id => { reblogs: true })
+        is_expected.to eq(target_account_id => { reblogs: true, notify: false })
       end
     end