Show forward-port batch retry-after on list

[xmo-odoo]

After an fwport failure I got really confused by the forwardbot's staunch decision to refuse to do anything no matter how many cron triggers I created.

Turns out this is because I added a 30mn interval between retries in b45ecf0 and I kinda didn't remember. And the list does not show anything so it looks like everything's gucci.

Hence the retry-after field should be displayed in the list view, maybe using a relative widget if that's available.