Make conversations titles consistent (no 'messages') - #4725

Have changed the text for the header bar's conversations icon (message_inbox_badge) and page title of the conversations page (message_inbox).

- The tooltip for the envelope icon in the header now reads 'Conversations'.
- The page title of the conversations page now reads 'Conversations – Inbox'. I thought it was worth keeping the word 'Inbox as a subtitle, because that is in effect what you are seeing.

What I haven't done yet is change the labels themselves, so they still read 'message_inbox_badge' and 'message_inbox'. If you think it would be a good idea to change them now to 'conversations_badge' and 'conversations_inbox' I can do that, but would need to know whether I should change the labels in the various locale files, or whether that is done via WebTranslateIt.