Divorce identity wipe from tryAgain functions

We'd like to live in a world where we can retry all the pending
conflicts in a conversation as a batch, which means we don't want to
wipe the identity key before processing each message. Thus, remove that
step from these handlers and encapsulate in a method on the conversation
model.

js/models/messages.js

@@ -47,6 +47,15 @@
             if (this.collection) {
                 return this.collection.conversation.contactCollection.get(this.get('source'));
             }
+        },
+        isOutgoing: function() {
+            return this.get('type') === 'outgoing';
+        },
+        getKeyConflict: function() {
+            return _.find(this.get('errors'), function(e) {
+                return ( e.name === 'IncomingIdentityKeyError' ||
+                         e.name === 'OutgoingIdentityKeyError');
+            });
         }
     });