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/views/stream_entries/_status.html.haml

@@ -16,7 +16,7 @@
           %strong= display_name(status.account)
         = t('stream_entries.reblogged')
 
-  = render partial: centered ? 'stream_entries/detailed_status' : 'stream_entries/simple_status', locals: { status: proper_status(status) }
+  = render partial: centered ? 'stream_entries/detailed_status' : 'stream_entries/simple_status', locals: { status: status.proper }
 
 - if include_threads
   = render partial: 'stream_entries/status', collection: @descendants, as: :status, locals: { is_successor: true }