/dmails: remove unused search[owner_id] param.

/dmails is restricted to viewing dmails for CurrentUser only (due to
Dmail.visible in the index action). Remove owner_id from subnavbar links
in /dmails, and don't support it in /dmails?search[owner_id], since it
doesn't actually do anything.

Also removes related dead methods and fixes tests that didn't test owner_id properly.

test/factories/post_disapproval.rb

@@ -0,0 +1,6 @@
+FactoryGirl.define do
+  factory(:post_disapproval) do
+    reason { %w(breaks_rules poor_quality disinterest).sample }
+    message { FFaker::Lorem.sentence }
+  end
+end