feat: Support variables in canned response (#6077)

- Added the option to insert variables in canned responses.
- Populate variables on selecting a canned response.
- Show a warning if there are any undefined variables in the message before sending a message.

app/javascript/dashboard/i18n/locale/en/conversation.json

@@ -132,6 +132,14 @@
           "PLACEHOLDER": "Emails separated by commas",
           "ERROR": "Please enter valid email addresses"
         }
+      },
+      "UNDEFINED_VARIABLES": {
+        "TITLE": "Undefined variables",
+        "MESSAGE": "You have {undefinedVariablesCount} undefined variables in your message: {undefinedVariables}. Would you like to send the message anyway?",
+        "CONFIRM": {
+          "YES": "Send",
+          "CANCEL": "Cancel"
+        }
       }
     },
     "VISIBLE_TO_AGENTS": "Private Note: Only visible to you and your team",