Make default conversation subject translatable and properly capitalize it

closes #6206, closes #6205

app/models/conversation.rb

@@ -83,7 +83,7 @@ class Conversation < ActiveRecord::Base
   end
 
   def subject
-    self[:subject].blank? ? "no subject" : self[:subject]
+    self[:subject].blank? ? I18n.t("conversations.new.subject_default") : self[:subject]
   end
 
   def subscribers(user)

config/locales/diaspora/en.yml

@@ -347,6 +347,7 @@ en:
     new:
       to: "To"
       subject: "Subject"
+      subject_default: "No subject"
       send: "Send"
       sending: "Sending..."
       abandon_changes: "Abandon changes?"