Add notifications for staging end

[xmo-odoo]

Sometimes as a user / admin you wait for the end of a staging for one reason or an other (e.g. do something on the branch).

Currently that requires eyeballing the duration of a staging then refreshing until it's actually done. The web notifications API might allow a more proactive method? Either hand-rolled or via the bus.

• register against a specific staging
• staging completes
• notification pops up in the UI
• yay