Improve counter caches on Status and Account (#7644)

Do not touch statuses_count on accounts table when mass-destroying
statuses to reduce load when removing accounts, same for
reblogs_count and favourites_count

Do not count statuses with direct visibility in statuses_count

Fix #828

spec/models/status_spec.rb

@@ -175,6 +175,13 @@ RSpec.describe Status, type: :model do
 
       expect(subject.reblogs_count).to eq 2
     end
+
+    it 'is decremented when reblog is removed' do
+      reblog = Fabricate(:status, account: bob, reblog: subject)
+      expect(subject.reblogs_count).to eq 1
+      reblog.destroy
+      expect(subject.reblogs_count).to eq 0
+    end
   end
 
   describe '#favourites_count' do
@@ -184,6 +191,13 @@ RSpec.describe Status, type: :model do
 
       expect(subject.favourites_count).to eq 2
     end
+
+    it 'is decremented when favourite is removed' do
+      favourite = Fabricate(:favourite, account: bob, status: subject)
+      expect(subject.favourites_count).to eq 1
+      favourite.destroy
+      expect(subject.favourites_count).to eq 0
+    end
   end
 
   describe '#proper' do