Add *duration* of staging to the main dashboard

[xmo-odoo]

When stagings fail, the page currently specifies when it was started, but not how long it lasted.

This causes two issues:

• it's harder to track the evolution of staging durations, or if staging durations suddenly shoot up
• when stagings are failing somewhat determininstically and we're trying to figure out which PR is causing it, it is harder to be on the ball and catch the staging right after the failure if we don't know when the failure happens (relative to staging start)