chore: enable coder inbox by default (#17077)

Add a flag to enable Coder Inbox by default, as well as supporting disabling the feature.

cli/testdata/coder_server_--help.golden

@@ -473,6 +473,10 @@ Configure TLS for your SMTP server target.
           Enable STARTTLS to upgrade insecure SMTP connections using TLS.
           DEPRECATED: Use --email-tls-starttls instead.
 
+NOTIFICATIONS / INBOX OPTIONS: 
+      --notifications-inbox-enabled bool, $CODER_NOTIFICATIONS_INBOX_ENABLED (default: true)
+          Enable Coder Inbox.
+
 NOTIFICATIONS / WEBHOOK OPTIONS: 
       --notifications-webhook-endpoint url, $CODER_NOTIFICATIONS_WEBHOOK_ENDPOINT
           The endpoint to which to send webhooks.

cli/testdata/server-config.yaml.golden

@@ -643,6 +643,10 @@ notifications:
     # The endpoint to which to send webhooks.
     # (default: <unset>, type: url)
     endpoint:
+  inbox:
+    # Enable Coder Inbox.
+    # (default: true, type: bool)
+    enabled: true
   # The upper limit of attempts to send a notification.
   # (default: 5, type: int)
   maxSendAttempts: 5