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.

app/models/status.rb

@@ -62,8 +62,12 @@ class Status < ApplicationRecord
     reply? ? :comment : :note
   end
 
+  def proper
+    reblog? ? reblog : self
+  end
+
   def content
-    reblog? ? reblog.text : text
+    proper.text
   end
 
   def target