Clarify label for starting a new conversation

When user a enters a number that is not a contact, we prompt them to start a new
conversation.

_locales/en/messages.json

@@ -835,9 +835,9 @@
       "message": "Hide menu bar",
       "description": "Label text for menu bar visibility setting"
     },
-    "newContact": {
-      "message": "Click to create new contact",
-      "description": ""
+    "startConversation": {
+      "message": "Start conversation…",
+      "description": "Label underneath number a user enters that is not an existing contact"
     },
     "newPhoneNumber": {
       "message": "Enter a phone number to add a contact.",

js/views/conversation_search_view.js

@@ -22,7 +22,7 @@
     },
     render_attributes() {
       return {
-        number: i18n('newContact'),
+        number: i18n('startConversation'),
         title: this.model.getNumber(),
         avatar: this.model.getAvatar(),
       };