Show update available in Site > Apps tab even if the bench is not updated

When a new change is pushed for an app, the "update available" banner is shown in the bench. This should be visible in site apps screen as well. Maybe allow me to deploy the bench from this screen itself.

Start at the Site > Apps screen and compare its current behavior with the existing update-available banner in the bench. Clarify whether this issue requires only showing the banner or also deploying the bench from that screen; done means the selected app's pending update is visible there and any agreed deployment flow works.

Written by the indexing model from the issue text.