Add Dropbox webhook integration.

This is just a basic Dropbox webhook integration. It just
notifies a user when something has changed, it does not
specify what changed. Doing so would require storing data,
as Dropbox API was created mainly for file managers, not
integrations like this.
Closes #5672

zerver/lib/integrations.py

@@ -284,6 +284,7 @@ WEBHOOK_INTEGRATIONS = [
         display_name='Desk.com',
         stream_name='desk'
     ),
+    WebhookIntegration('dropbox', ['productivity'], display_name='Dropbox'),
     WebhookIntegration('freshdesk', ['customer-support']),
     GithubIntegration(
         'github',