Add support for private pinned posts (#16954)

* Add support for private pinned toots

* Allow local user to pin private toots

* Change wording to avoid "direct message"

spec/models/status_pin_spec.rb

@@ -24,11 +24,11 @@ RSpec.describe StatusPin, type: :model do
       expect(StatusPin.new(account: account, status: reblog).save).to be false
     end
 
-    it 'does not allow pins of private statuses' do
+    it 'does allow pins of direct statuses' do
       account = Fabricate(:account)
       status  = Fabricate(:status, account: account, visibility: :private)
 
-      expect(StatusPin.new(account: account, status: status).save).to be false
+      expect(StatusPin.new(account: account, status: status).save).to be true
     end
 
     it 'does not allow pins of direct statuses' do