DRY up reblog vs original status check

Checking reblog vs original status was happening in multiple places
across the app. For views, this logic was encapsulated in a helper
method named `proper_status` but in the other layers of the app, the
logic was duplicated.

Because the logic is used at all layers of the app, we extracted it into
a `Status#proper` method on the model and changed all uses of the logic
to use this method. There is now a single source of truth for this
condition.

We added test coverage to untested methods that got refactored.

spec/models/status_spec.rb

@@ -97,4 +97,15 @@ RSpec.describe Status, type: :model do
   describe '#favourites_count' do
     pending
   end
+
+  describe '#proper' do
+    it 'is itself for original statuses' do
+      expect(subject.proper).to eq subject
+    end
+
+    it 'is the source status for reblogs' do
+      subject.reblog = other
+      expect(subject.proper).to eq other
+    end
+  end
 end