/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.

app/models/dmail.rb

@@ -86,18 +86,6 @@ class Dmail < ActiveRecord::Base
   end
   
   module SearchMethods
-    def for(user)
-      where("owner_id = ?", user)
-    end
-
-    def inbox
-      where("to_id = owner_id")
-    end
-
-    def sent
-      where("from_id = owner_id")
-    end
-
     def active
       where("is_deleted = ?", false)
     end
@@ -139,10 +127,6 @@ class Dmail < ActiveRecord::Base
         q = q.search_message(params[:message_matches])
       end
 
-      if params[:owner_id].present?
-        q = q.for(params[:owner_id].to_i)
-      end
-
       if params[:to_name].present?
         q = q.to_name_matches(params[:to_name])
       end